Russia’s Foreign Ministry has complained to the U.S. about Washington’s refusal to let Russian inspectors take part in an investigation into a chemical weapon attack in Syria that left dozens of civilians dead earlier this month. Foreign Minister Sergei Lavrov made the comments in a phone conversation with U.S. Secretary of State Rex Tillerson, the ministry said Friday. The April 4 gas attack, in which investigators say they believe Sarin was used, ratcheted up tensions between Russia and the U.S., which launched air strikes on Syria as a result of the attack. Russia has defended Damascus and blamed Syrian rebels for the attack. While Russia’s Foreign Ministry said Tillerson had agreed with Lavrov to consider an “objective investigation” into the attack, the U.S. State Department said Tillerson had expressed support for the Organization for the Prohibition of Chemical Weapons’ current investigation. Lavrov also used the phone call to demand that the U.S. side return “Russian diplomatic property in the USA unlawfully confiscated by the Barack Obama administration.” Shortly before Trump took office, then-President Obama expelled 35 Russian diplomats and ordered them out of two vacation retreats near Washington and New York that he said were linked to espionage activities.