British Foreign Secretary William Hague said there were mounting claims that Syrian military forces have deployed chemical weapons.
"The use of chemical weapons is a war crime," he said in a statement. "As an urgent first step, we will work with our partners to increase the pressure on the regime to allow unrestricted access for the U.N. team to investigate on the ground in Syria."
The Syrian government requested a U.N. investigation into the alleged use of chemical weapons by rebel forces. The government balked out of concern over the scope of any eventual investigation, however.
The United States and some of its allies said recently that there's preliminary evidence to suggest Syrian forces may have used small amounts of chemical weapons, specifically the nerve agent sarin.
The BBC reports Thursday that it's seen evidence that appears to verify claims that Syrian forces used chemical weapons in northern Syria last month. At least two canisters filled with poisonous gas were allegedly dropped from helicopters on areas where civilians were present.
The BBC reports that samples from the area and from alleged victims were on their way to foreign governments for testing.
The U.N. General Assembly passed a resolution Wednesday condemning the Syrian government's use of heavy weapons against civilians, the fifth such resolution since the uprising began in 2011. The resolution described the National Coalition for Syrian Revolutionary and Opposition Forces as "effective representative interlocutors needed for a political transition."