The U.S. ambassador to the United Kingdom on Friday called out Russia for "lying" in accusing Britain of staging an apparent chemical weapon attack in Syria.

Ambassador Woody Johnson took aim at Russia's Defense Minister Sergey Lavrov for suggesting, without evidence, that the alleged chemical bombing was a "fabrication" by a U.K.-backed intelligence agency working to smear Russia.

"Russian Defense Minister's accusations against the UK are absurd. Time for Russia to stop lying and start meeting its international obligations," Johnson tweeted Friday, nearly a week after the bombing of a hospital near the Syrian capital of Damascus killed over 40 people.

Lavrov has claimed that Russia had "irrefutable information" proving that photographic evidence of the bombing was a hoax. The Russian military maintains that the attack was "staged and directed by Britain," according to The Associated Press.

U.K. officials on Friday denied the accusations, with U.K. Ambassador to the United Nations Karen Pierce calling it a "grotesque" and "blatant lie," according to the BBC.

The bombing, which the U.S. believes was perpetrated by the ruling regime in Syria, appears to be a chemical attack, an assessment based in part on photos of people struggling with its effects. The use of a chemical agent has not yet been certified by an international agency.

Russia has supported Syrian President Bashar Assad in Syria amid the country's ongoing civil war. The U.S., U.K. and France believe Assad is behind the attack.

Tensions between Russia and Britain have escalated in recent weeks as both countries have expelled diplomats from each other's nations over the apparent poisoning of a former Russian double agent on British soil.

Russia has claimed no responsibility for the poisoning and suggested that the U.K. abducted the ex-spy's daughter in order to use her to place blame on the Kremlin for her poisoning. The U.S. has also accused Russia of poisoning the two using a nerve agent found at their home.