Russian President Vladimir Putin and German Chancellor Angela Merkel have discussed the situation in Syria and a controversial Baltic gas pipeline project in a telephone conversation, the Kremlin says.

Putin criticized the air strikes carried out late last week by the United States, Britain, and France against Syrian government facilities, the Kremlin said in a statement on April 17.

He told Merkel that strikes were an "act of aggression" that violated the UN Charter and "dealt a substantial blow" to efforts to end the seven-year war in Syria, it said.

The United States says the April 14 strikes targeted the Syrian government's chemical-weapons infrastructure following a suspected poison-gas attack that killed dozens of civilians in what was then the rebel-held town of Douma, near Damascus, on April 7.

The suspected attack and the air strikes added to already severe strains in Russia's relationship with the West, which has been badly frayed by issues including Moscow's interference in Ukraine and its backing for President Bashar al-Assad throughout the war in Syria, which has killed hundreds of thousands of people and displaced millions.

According to the Kremlin, Putin and Merkel both stressed the importance of an "objective investigation" into the suspected attack in Douma by inspectors from the Organization for the Prohibition of Chemical Weapons (OPCW). Russia claims there was no chemical-weapons attack but has not provided evidence of that assertion.

Putin and Merkel also discussed the Nord Stream 2 project, a pipeline running from Russia through the Baltic Sea to Germany, the Kremlin said.

Critics say the pipeline would increase European dependence on Russia for energy and enrich its state-owned energy companies at a time when Moscow stands accused of endangering European security through alleged election meddling and other actions.

The United States, Poland, and the Baltic states oppose the project.

Merkel appeared to signal a decrease in support for the planned pipeline last week, saying on April 10 that there were "political considerations" to take into account.

She said that she had told Putin by phone that the project could not go ahead without clarity on Ukraine's role as a gas transit route.

Nord Stream 2 would decrease Russia's reliance on Ukraine as a route for pumping its gas further West, into the European Union.