'It's game changing': President Barack Obama says US has evidence that President Bashar Assad is using chemical weapons in Syria

So what happens now that he believes the 'red line' has been crossed? John Hall reports

President Barack Obama has revealed there is now “some evidence” that chemical weapons were used inside Syria, an act he has previously called a “red line” for potential US intervention in the country.

At a news conference this afternoon the US president stated that although he doesn't know how or when they were used, new evidence has emerged that forms the strongest proof yet that chemical weapons have been deployed during the ongoing civil war.

Obama told reporters that he must now gather “all the facts” before deciding how to react, but repeated his statement that the use of chemical weapons would be “game changing” with "enormous consequences" in the region.

He specifically stated that there is “some evidence” that President Bashar Assad's government had used chemical weapons against his own people but made it clear that that is only a preliminary assessment based on intelligence information.

Obama went on to say that he has asked Pentagon for range of options if it is eventually proved beyond doubt that Assad used chemical weapons, adding that the international community has to be completely confident in the assessment before taking any action.

Obama said: “We don't have a chain of custody that establishes [exactly what happened],” before adding “[If it emerges that Syria used chemical weapons] we would have to rethink the range of options that are available to us.”

He added: “Obviously there are options to me that are on the shelf right now that we have not deployed,“ he said, noting that he had asked Pentagon planners last year for additional possibilities.

Obama’s talk of “red lines” and “enormous consequences” echoes the sentiments of Britain, France and Israel.

In Tehran, Iranian Foreign Minister Ali Akbar Salehi said that use of chemical weapons in Syria would also be a “red line” for Iran, but suggested rebel forces should be investigated rather than the Islamic Republic's allies in Damascus.

The developments come just hours after 13 people were killed by a huge bomb blast in the Syrian capital Damascus, sowing fear and chaos in a busy commercial district of the capital for the second consecutive day.

Yesterday the Syrian prime minister narrowly escaped an assassination attempt after a car bomb struck near his convoy, a few miles away from today’s blast.

The bombings appear to be part of an accelerated campaign by opposition forces seeking to topple President Bashar Assad to strike at his heavily protected seat of power.

“I heard a very loud bang and then the ceiling collapsed on top of me,” said Zaher Nafeq, who owns a mobile phone shop in the Damascus Towers, a 28-floor office building. He was wounded in his hand and his mobile phone shop was badly damaged in the blast.

Syrian TV said Tuesday's explosion was caused by a “terrorist bombing” in the commercial district of Marjeh. Assad's regime refers to opposition fighters as “terrorists.”

There was no immediate claim of responsibility for the blast, which the TV said also wounded 70 people. Car bombs and suicide attacks targeting Damascus and other cities that remain under government control in the third year of the conflict have been claimed in the past by the al-Qaida affiliated Jabhat al-Nusra group — one of scores of rebel factions fighting to oust Assad.

The target of Tuesday's attack was not immediately clear, although the explosion took place outside the former Interior Ministry building that was also damaged in the blast.

Ambulances rushed to the scene and Syrian state TV aired footage of fire trucks in central Marjeh Square and firefighters trying to extinguish a blaze that engulfed several cars and buildings. A man was seen lying on the ground in a pool of blood while another, apparently wounded, was seen being carried by civilians into a bus.

Inspecting the site of the blast, Interior Minister Mohammed al-Shaar, who himself escaped a car bomb that targeted his convoy in December, told reporters the back-to-back attacks in the capital were in response to the “victories and achievements scored by the Syrian Arab Army on the ground against terrorism.”

The government has waged an offensive in recent weeks, overrunning two rebel-held Damascus suburbs and a town outside the capital. Regime troops have also captured several villages near the border with Lebanon as part of their efforts to secure the strategic corridor running from Damascus to the Mediterranean coast, which is the heartland of the president's Alawite sect, an offshoot of Shiite Islam.

The Britain-based Syrian Observatory for Human Rights said 14 people were killed and 65 wounded in Tuesday's blast. The group, which relies on a network on activists based in Syria, often gives different casualty tolls from the figures released by the government. The discrepancy could not immediately be reconciled.

Local residents said they heard gunfire in the area of the attack immediately after the 11 a.m. blast, apparently meant to disperse people and open the way for ambulances. The residents spoke to The Associated Press on condition of anonymity, fearing reprisals from authorities for speaking to reporters.

The explosion underlined the tenuous security in the Syrian capital, just a day after a remotely detonated roadside bomb struck Prime Minister Wael al-Halqi's convoy. The premier escaped the assassination attempt unharmed, according to state TV.

But a government official said two people were killed and 11 wounded in the assassination attempt. The official spoke to the Associated Press news agency on condition of anonymity because he was not authorized to brief the media.

The Observatory said Monday's bombing killed at least five people, including two of al-Halqi's bodyguards and one of the drivers in his convoy.

The bombings laid bare the vulnerability of Assad's regime and highlighted an accelerating campaign targeting government officials, from mid-level civil servants to the highest echelons of the Syrian regime.

Syria's conflict began with largely peaceful anti-government protests in March 2011, but has since morphed into a civil war that has killed more than 70,000 people, according to the United Nations.

Earlier today, Syrian troops battled opposition fighters near a military helicopter base in the northern province of Aleppo, killing 15 rebels in a single airstrike against their positions, according to Observatory director Rami Abdul-Rahman.

The rebels tried to storm the Mannagh base late Monday but the regime deployed fighter jets to the area, pounding rebel positions around the base, which is near Syria's border with Turkey, Abdul-Rahman said.

He added that fighting was also raging today around other airports in the country, including the Damascus International Airport just south of the capital.

In the north, rebels besieged military airport facilities, including Kweiras northeast of Aleppo city and the Nairab military base, adjacent to Aleppo's civilian airport, the country's second largest.

In neighbouring Idlib province, a government airstrike near the Bab al-Hawa border crossing with Turkey killed one child and wounded several more people, the Observatory said.

An Associated Press journalist near Reyhanli on the Turkish side of the frontier described a huge plume of black smoke and reported seeing wounded people being rushed by ambulance from the Syrian side to the Turkish control point.

Thousands of Syrian refugees live in a makeshift camp known as the Bab al-Hawa refugee camp near the border with Turkey.

Air power has proven to be Assad's greatest advantage in the civil war, and he has exploited it to push back rebel advances and prevent the opposition from setting up a rival government in the territory it has seized in the north.