US Vice President Joe Biden arrived in Kiev to reassure its pro-Western leaders that Washington remains committed to Ukraine despite stepped-up efforts to work with Russia against Islamic State jihadists.
Biden's three-day visit is his fourth to Kiev since Russia annexed Ukraine's Crimea peninsula in March 2014 and then watched with approval as pro-Kremlin insurgents carved out their own region in the eastern industrial heartland of the ex-Soviet state.
After arriving at around midnight (2200 GMT yesterday), he is due to meet Ukrainian President Petro Poroshenko today and deliver a highly-anticipated address to parliament the following day.
"We do not know if there is any other historical precedent for a foreign official giving a speech like this," a senior US administration official, speaking on condition of anonymity, said in a teleconference with reporters.
Washington and Kiev's EU allies support Ukraine's view of Russia being an "aggressor" that orchestrated the separatist revolt in reprisal for the February 2014 ousting of a Moscow-backed president -- an assertion the Kremlin denies.
Both the US and EU have slapped stiff economic sanctions on Russian President Vladimir Putin's inner circle and have helped train and equip Ukraine's dilapidated and underfunded army with defensive equipment such as advanced radar.
But the situation changed when Russia launched ferocious air strikes against Syrian President Bashar al-Assad's enemies on September 30.
Washington accused Putin of trying to prop up his most important Middle East ally by targeting Western-backed Syrian rebels instead of IS and other extremists holding parts of Syria and Iraq.
Yet IS's claim that it downed a Russian airliner carrying 224 holidaymakers and crew from Egypt on October 31 appears to have prompted Moscow to focus more on bombing oil infrastructure and other jihadist targets.
The November 13 Paris attacks further prompted French President Francois Hollande to try to enrol Russia in a "grand coalition" against IS including the US and some European and Arab states.
Hollande's mission has been treated with caution by the White House and overt fright by Ukraine, a country of about 40 million.
The senior US official said Biden would take extra care to stress that the overtures toward Putin in no way affected the West's support for Kiev.
"I think that is going to be a major theme of the trip -- that nothing that is going on in the Middle East has changed one iota of our commitment to the Ukrainian people and to their security," the US official said.
Yet Biden arrives in a country whose morale is sagging due to Poroshenko's seeming inability to erase the corruption that has ravaged Ukraine for much of its recent history.