Russian Foreign Minister Sergey Lavrov has told U.S. Secretary of State Rex Tillerson that Russia regrets U.S. opposition to Russian inspectors taking part in an investigation into a chemical weapons attack in Syria.

The Foreign Ministry said in a statement that Lavrov spoke with Tillerson by telephone on Friday and they agreed once again to look into organizing an objective investigation under the aegis of the Organization for the Prohibition of Chemical Weapons.

The ministry said Lavrov also repeated Moscow's demand that the U.S. return Russia's estates in the states of Maryland and New York.

The two retreats were shut down by the Obama administration late last year in retaliation for Russia's cyber-meddling in the presidential election. The U.S. said the estates were being used for intelligence activities.