As US President Barack Obama announced sanctions against Russia last Monday, a group of Russian officials arrived in San Francisco for a surprise inspection of the US strategic nuclear arsenal.
The inspections, part of a process agreed to under the 2010 New START weapons reduction treaty, signalled business as usual in at least one area of bilateral co-operation.
The treaty is far from the only arms-control investment between the two countries. Russia and the United States have co-operated on efforts to keep nuclear materials out of terrorist hands, they are jointly negotiating limitations on Iran's nuclear programme and they have joined in the effort to destroy Syria's chemical weapons arsenal.
So far, all of those efforts have been off limits in the dispute over Ukraine that has sent US-Russia relations into a steep, downward spiral.
US administration officials have been quick to emphasise that they would like to keep it that way, and have said they believe Russia feels the same way.
Russia is key to the non- proliferation agenda that has been Obama's signature foreign policy initiative. Asked on Friday whether Moscow intends to remain co-operative, National Security Adviser Susan Rice said: "We haven't seen any evidence to the contrary."
But the Russian approach to international comity and norms has been uneven. Late last week, even as it officially annexed Crimea while the rest of the world cried foul, Moscow facilitated Ukrainian overflights of Russian troop emplacements on their shared border, under the 1992 Open Skies Treaty that is part of the world's intricate system of security arrangements.
Under New START, Russia and the United States each agreed to reduce their strategic warheads to 1,550 by 2018. Russia shunned an offer by Obama last year to negotiate further reductions, in part over its objections to US missile defence plans in eastern Europe, but it continues to abide by provisions of the treaty.
Those include 18 surprise inspections by both sides each year. The teams do not have to say which military installation they want to visit or reveal what they want to see until they arrive. The surprise element, a senior administration official said, was included so that "there's no chance to stash anything away or move it out of the line of sight. That's a very important part of the treaty."
The most recent US inspection in Russia took place during the last week of February, just as the administration said that Russian troops were beginning to pour into Crimea.
The treaty has a clause that allows either side to withdraw, and the senior official said that "if the Russians at some point decided they wanted to exercise" that right, they could do so.
"But I hope they would think very long and hard about that," the official said.
This week, Obama travels to the Netherlands for a summit on nuclear security, the third in a series he initiated in a major speech during his first year in office aimed at preventing nuclear terrorism.
A Russian delegation will be headed by Foreign Minister Sergey Lavrov. One key to the summit's success will be the ability of all involved to separate its agenda from the series of international meetings on the Ukraine crisis that Obama will attend immediately afterwards in Belgium, from which the Russians pointedly will be excluded.
"We haven't seen any evidence so far" that Russia will use the summit to express unhappiness over Ukraine, the senior official said.
The extent to which Russia is willing to separate Ukraine from other issues in which the United States and its partners have major stakes arose last week at the end of the latest round of talks in Vienna over Iran's nuclear programme.
In a statement on Wednesday at the end of the latest round, Russian Deputy Foreign Minister Sergey Ryabkov, his country's chief envoy to the talks, warned that Moscow might use them to take "retaliatory measures" against Western sanctions.