Senior U.S. and Russian diplomats met Monday at the State Department to discuss “irritants” between the two countries, as the Kremlin stepped up its demand that the White House return two diplomatic compounds seized last year as punishment for Moscow's interference in the 2016 election.

The mansions, one on Maryland's Eastern Shore and the other outside New York City, were high on the agenda of Deputy Foreign Minister Sergei Ryabkov when he spent more than three hours in talks with under Secretary of State for Political Affairs Thomas Shannon in Washington.

In advance of Ryabkov’s arrival at Foggy Bottom along with Ambassador Sergey Kislyak, angry Russian officials issued a litany of complaints about the property seizure in December and the continuing ban on Russian diplomats being allowed to visit the country houses they used as weekend retreats.

Kremlin spokesman Dmitry Peskov called the seizure “absolutely in breach of international law,” and said it was unacceptable for the U.S. to set any preconditions for returning the mansions. Russian Foreign Minister Sergei Lavrov characterized the move as “robbery in broad daylight,” and opined that “Decent and well brought-up people do not behave in such a way.”

“How can you seize property which is protected by a bilateral, intergovernmental, ratified document and, to return it, act according to the principle ‘What is mine is mine, and what is yours we’ll share’?” he added.

(Elyse Samuels/The Washington Post)

As Ryabkov left the State Department at 5:45 p.m., a reporter shouted out asking whether the issue of the two “dachas,” as the Russians call them, had been resolved.

“Almost, almost,” Ryabkov replied before getting into his waiting limousine.

On Tuesday, the State Department said the talks were “tough, forthright, and deliberate, reflecting both parties’ commitment to a resolution.”

“The United States and Russia seek a long-term solution that would address areas of bilateral concern that have strained the relationship,” State Department spokeswoman Heather Nauert said in a statement. “The talks reflected a spirit of goodwill, but it is clear that more work needs to be done.”

The issue of the seized Russian buildings has been a source of growing friction in recent months, as the Trump administration has sought to improve relations with Russia even while it juggles investigations into Russian attempts to meddle in the presidential election.

Shannon and Ryabkov were scheduled to meet last month in St. Petersburg, but Moscow abruptly canceled after 38 individuals and groups were added to a list of sanctions related to Russian activity in eastern Ukraine. U.S.- Russia relations are now at a post-Cold War low and the Trump administration thinks it is dangerous if they remain so toxic. The talks between senior diplomats are designed to build trust and set the stage for cooperation on bigger issues like the war in Syria.

Washington has its own list of complaints. It wants Moscow to stop harassing U.S. diplomats and to lift a ban on Americans adopting Russian children.

A Russian diplomatic compound, seized by the United States in December 2016 in connection with suspected Russian hacking activities, in Upper Brookville, New York. (Andrew Gombert/EPA)

But the dispute over the compounds risks derailing larger goals. On his visit to Belarus, Lavrov said “anti-Russian feeling” in the United States could interfere with Moscow and Washington cooperating on other issues.

Secretary of State Rex Tillerson has suggested the United States is amenable to returning the properties, which were discussed by President Trump and Russian President Vladimir Putin at their first in-person encounter at the Group of 20 summit in Hamburg this month. Russia has threatened retaliation if it doesn't get its dachas back. Kremlin officials have suggested they could seize property owned by the U.S. embassy in Moscow or deport dozens of American diplomats in payback for the U.S. expulsion of 35 Russian “intelligence operatives” when it took over the Maryland and New York mansions.

“It is best to immediately return our property, otherwise Russia has the right to a tit-for-tat response in relation to American property in Russia,” Russian Foreign Ministry spokeswoman Maria Zakharova said last month. “I want to confirm that the retaliatory measures are in the works.