Vice President Joe Biden spoke with Ukraine's president Tuesday after pro-Russian insurgents downed an army helicopter against ceasefire orders from their commander, the White House said.

The chopper downing, in which nine Ukrainian soldiers were killed, could torpedo hopes that Ukraine's Western-backed President Petro Poroshenko -- having ordered a one-week unilateral ceasefire Friday that the rebels accepted on Monday -- will be able to negotiate an end to 11 weeks of violence that has claimed 435 lives, according to U.N. figures and an Agence France Presse count.

Nine Ukrainian servicemen died on Tuesday when pro-Russian insurgents shot down an army helicopter in the separatist east, which prompted Ukraine's new Western-backed president to warn that he may revoke his one-week unilateral ceasefire to allow government forces to retaliate for the incident.

"There were nine people on board the helicopter. According to preliminary information... everyone on board died," Ukrainian defense spokesman Vladyslav Seleznyov wrote in a Facebook post.

Russian President Vladimir Putin on Tuesday urged Kiev to extend the shaky truce with insurgents in eastern Ukraine and launch talks despite them shooting down a helicopter with nine on board.

The Russian strongman said he had asked senators to rescind a resolution allowing him to invade Ukraine in order to encourage a "peace process" but vouched at the same time to always protect the interests of Russians in the neighboring country.

Ukraine's new Western-backed president faced pressure on Tuesday to negotiate with top rebel commanders after a surprise turnabout in which the insurgents agreed to a truce and talks on ending their pro-Russian uprising.

The prominent head of the self-declared Donetsk People's Republic on Monday unexpectedly reversed his firm rejection of President Petro Poroshenko's peace overtures by agreeing to a ceasefire that would last until Friday morning.

NATO foreign ministers convene Tuesday to discuss the Iraq crisis after U.S. Secretary of State John Kerry pledged "intense" support to Baghdad in the fight against militants pushing towards the capital.

Kerry made a surprise visit to Iraq on Monday as Sunni insurgents led by the jihadist Islamic State of Iraq and the Levant, or ISIL, seized a strategic town in the northern part of the country, while security forces retook a border crossing with Syria.

U.S. President Barack Obama on Monday warned Vladimir Putin that Russia would face new sanctions if it failed to both stop the flow of weapons into Ukraine and halt support for separatists.

The White House said that Obama delivered the warning in a telephone call with the Russian leader, in which he called for "concrete actions" by Moscow to de-escalate the situation.

European Union foreign ministers urged Russia on Monday to back the Ukraine government's peace plan or face tougher sanctions.

The 28 EU ministers were also seeking a policy response on Iraq and examining fallout from the Syrian conflict that is destabilizing the Middle East, driving a refugee exodus washing up on EU shores.

Ukraine's pro-Russian insurgents agreed for the first time on Monday to a temporary ceasefire and talks with the new Western-backed president on ending 10 weeks of fighting that have threatened the very survival of the ex-Soviet state.

The surprise announcement from the head of the self-proclaimed Donetsk People's Republic came a day after Russian President Vladimir Putin publicly threw his weight behind Kiev's peace overtures and urged the separatists to halt fire.

Russian President Vladimir Putin on Sunday called on Ukraine's leadership and rebels to start genuine dialogue, saying Kiev should halt military operations and guarantee the rights of Russian speakers in the separatist east.

"It's necessary to start detailed, substantial dialogue," Putin told reporters on Sunday, adding Moscow would support a peace plan drawn up by the Ukrainian leader.

Ukraine's new Western-backed leader and Russian President Vladimir Putin both called for dialogue Sunday to end a pro-Moscow uprising that has threatened the ex-Soviet state's survival and brought Europe to the edge of all-out war.

The twin calls from the central figures of the worst East-West crisis since the Cold War era both came with conditions and the ragtag militias in Ukraine's eastern rustbelt showing no desire to end their independence drive.