David Cameron and Barack Obama have said any threat to deploy chemical weapons by the Syrian regime would be "completely unacceptable".
According to Downing Street, the two leaders, who spoke on Wednesday evening, agreed that any recourse to chemical weapons by Syrian leader Bashar al-Assad, would force them to “revisit their approach” to the crisis.
The two leaders said there was "much more to do" to stop the brutal killing of civilians in the Middle East state.
The threat came just hours after Syrian forces shelled several Damascus suburbs ahead of house-to-house raids by regime troops. According to the Associated Press, at least 35 suspected rebels were killed in the raids.
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Earlier this week President Obama warned that any movement of Syria's stockpile of chemical weapons would be a "red line" which would have "enormous consequences".
Cameron also discussed the situation with French President Francois Hollande and the two European nations agreed to "work more closely to identify how they could bolster the opposition and help a potential transitional Syrian government after the inevitable fall of Assad".
Downing Street said Cameron and President Obama "both agreed that the use - or threat - of chemical weapons was completely unacceptable and would force them to revisit their approach so far".
A spokeswoman added: "The Prime Minister restated the risk to the wider region posed by the fighting and the fact that regional and international cooperation was vital.
"He reinforced the need to work in close cooperation with Turkey, Qatar, Saudi Arabia and others on the issue."
Cameron also spoke to French President Francois Hollande (pictured) about the Syrian crisis
They also "firmly agreed that there was much more to do in order to stop the brutal killing of civilians and help put Syria on a path towards peace and stability".
They discussed how to build on the support both nations had offered to the opposition in Syria "to end the appalling violence and bring about stability".
The leaders said they hoped a meeting of opposition groups in Cairo would show "real unity of purpose and coherence in working towards transition".
Cameron and Obama agreed that more should be done by the international community to channel humanitarian aid to Syrian refugees through the UN appeal.
Cameron and Hollande discussed the need to maintain international pressure on the Assad regime.
They agreed that the refugee situation was "deeply troubling" and Hollande said that the humanitarian crisis would be the focus of France's United Nations Security Council ministerial meeting at the end of the month.
The pair welcomed the appointment of Lakhdar Brahimi as the new UN envoy and hoped that he would carry on the work of Kofi Annan in "seeking a credible political solution - as well as holding the regime to account for any further atrocities".
09/09/2012 12:25 PM EDT
Syria Blasts Aleppo By Air
09/05/2012 7:50 AM EDT
Syrian City In Flames
08/25/2012 1:45 PM EDT
Lebanese Pilgrim Freed
Hussein Ali Omar, 60, one of 11 Lebanese Shiite pilgrims that Syrian rebels have been holding for three months in Syria, hugs his mother, right, upon arrival at his house in the southern suburb of Beirut, Lebanon, 2012. Syrian rebels freed Omar on Saturday in a move aimed at easing cross-border tensions after a wave of abductions of Syrian citizens in Lebanon. The Shiite pilgrims were abducted May 22 after crossing into Syria from Turkey on their way to Lebanon. (AP Photo/Hussein Malla)
08/24/2012 12:22 PM EDT
This image made from video and released by Shaam News Network and accessed Thursday, Aug. 23, 2012, purports to show the funeral of children in Daraya, near Damascus, Syria. Syrian troops backed by tanks and helicopters broke into a Damascus suburb on Thursday following two days of shelling and intense clashes as part of a widening offensive by President Bashar Assad's forces to seize control of parts of the capital and surrounding areas from rebel fighters, activists said. At least 15 people were killed in the offensive on Daraya, only a few miles (kilometers) southwest of Damascus. (AP Photo/Shaam News Network SNN via AP video)
08/24/2012 11:05 AM EDT
Lebanon Sees Heaviest Clashes In Months
Clashes between Assad supporters and opponents of the Syrian regime killed two people in Lebanon on Friday, the Associated Press reports. 17 people were injured.
The AP gives more context:
Syria was in virtual control of its smaller neighbor for many years, posting tens of thousands of troops in Lebanon, before withdrawing under pressure in 2005. Even without soldiers on the ground, Syria remains influential, and its civil war has stirred longstanding tensions that have lain under Lebanon's surface.
Read more on HuffPost World.
08/24/2012 11:02 AM EDT
A Sunni gunman fires a gun during clashes that erupted between pro and anti-Syrian regime gunmen in the northern port city of Tripoli, Lebanon, Friday, Aug. 24, 2012. The latest round of fighting first erupted on Monday in northern Lebanon and at least 15 have been killed in Tripoli this week and more than 100 have been wounded in fighting that is a spillover from Syria's civil war. (AP Photo/Hussein Malla)
08/24/2012 11:00 AM EDT
Refugee Numbers Soar
@ KenRoth :
UN reports 200,000 #Syria refugees, 30,000 in past week alone. Many more internally displaced not counted. http://t.co/BaM6u59j
08/23/2012 2:00 PM EDT
Syrian boy Musataf Alhafiz, 11, who fled his home with his family due to fighting between the Syrian army and the rebels, carries his brother Saif, 9 months, while he and others take refuge at the Bab Al-Salameh border crossing, in hopes of entering one of the refugee camps in Turkey, near the Syrian town of Azaz, Thursday, Aug. 23, 2012. Thousands of Syrians who have been displaced by the country's civil war are struggling to find safe shelter while shelling and airstrikes by government forces continue. (AP Photo/Muhammed Muheisen)
08/23/2012 12:09 PM EDT
Heaviest Bombardment This Month
Helicopter gunships shelled Damascus on Wednesday as Syrian security forces intensified their assault on the capital. Activists report that at least 47 people were killed.
"The whole of Damascus is shaking with the sound of shelling," a woman in the neighborhood of Kfar Souseh told Reuters.
Read more on HuffPost World.