Foreign Secretary William Hague has urged Syrian president Bashar Assad to allow the United Nations to investigate fresh reports that chemical weapons have been used in the country's bloody conflict.
Mr Hague said Assad's refusal to allow a UN team access to Syria showed he was leading a regime that was "determined to conceal the truth" about what was happening.
The BBC reported that eyewitnesses in Saraqeb in northern Syria saw government helicopters drop at least two devices containing poisonous gas during an assault last month.
The Government has previously said it has "limited but credible" evidence showing that chemical weapons have been used in Syria.
Mr Hague said: "New reports have emerged of further chemical weapons use by the regime in Syria. The Assad regime continues to refuse access to the country to UN investigators.
"This makes it difficult to corroborate these reports, and confirms that this is a regime determined to conceal the truth about what is happening in the country.
"We have previously stated that we have limited but credible evidence from various sources showing chemical weapon use in Syria. We have briefed our allies, partners and the UN on this evidence and we are working actively to get more information.
"The use of chemical weapons is a war crime. Those who order the use of chemical weapons, and those who use them, should be in no doubt that we will work in every way we can to hold them to account.
"As an urgent first step, we will work with our partners to increase the pressure on the regime to allow unrestricted access for the UN team to investigate on the ground in Syria."