The shaky ceasefire in Ukraine was thrown into peril Tuesday when pro-Moscow separatists shot down a Ukrainian military helicopter, killing nine servicemen.

Ukrainian President Petro Poroshenko warned he may end the weeklong truce ahead of time. The deadly attack came a day after the rebels vowed to respect the ceasefire, which began last Friday.

Russian President Vladimir Putin, meanwhile, played the peacemaker, urging Ukraine to extend the truce and sit down for talks with the rebels. He also moved to rescind a parliamentary resolution authorizing him to use the Russian military in Ukraine.

Poroshenko declared the ceasefire as part of a plan to end two months of fighting between government troops and pro-Russian insurgents in the east. The violence, which erupted after the Kremlin's lightning annexation of Crimea in March, has left hundreds dead.

The ceasefire has been repeatedly broken by sporadic clashes, and it was violated again Tuesday when rebels used a shoulder-fired missile to down a helicopter in Slovyansk, a key flashpoint in the insurgency.

Poroshenko said in a statement that the insurgents had fired on Ukrainian positions 35 times since the ceasefire was announced, and he instructed Ukrainian soldiers to fire back "without hesitation" if attacked.

"The president doesn't exclude that the ceasefire could be lifted ahead of time, taking into account its constant violation by the rebels controlled from abroad," Poroshenko's office said.

It added that Poroshenko expects that his phone call, set for Wednesday, with Putin, German Chancellor Angela Merkel and French President Francois Hollande would lead to "practical steps by Russia to disarm and recall mercenaries from Ukraine and ensure the reliable protection of the border."

The U.S. and NATO have accused Russia of supporting the rebellion in the east with troops and weapons. Moscow has denied that, saying Russian citizens who joined the insurgents were acting on their own.

Putin calls on Ukraine to extend ceasefire

During a trip Tuesday to Vienna, Putin employed a combination of pressure and goodwill intended to protect Moscow's interests without triggering heavier Western sanctions, which are likely to be a topic of discussion during a European Union summit on Friday.

'We hope that the ceasefire will be extended and it will be used for substantive talks.'- Vladimir Putin, Russian president

Putin said that the Ukrainian demand that the insurgents lay down their weapons within a week was unrealistic, explaining that they would be reluctant to disarm for fear of government reprisals.

He called for a longer truce to be accompanied by peace talks, arguing that they should lead to constitutional amendments and other legal changes that would protect the rights of Russian-speakers in the east.

"We hope that the ceasefire will be extended and it will be used for substantive talks," Putin said.

Speaking shortly after the helicopter was shot down, Putin blamed Ukrainian forces for breaking the ceasefire on Tuesday by launching an airborne raid in Slovyansk. He did not mention the downing of the helicopter.

Earlier Tuesday, the Russian president asked parliament to cancel a March 1 resolution sanctioning the use of military force in Ukraine, a move Poroshenko praised as a "practical step" toward peace. The pliant upper house is expected to rubber-stamp the move Wednesday.

U.S., European powers mull more sanctions

White House spokesman Josh Earnest said the U.S. is "reserving judgment" about Putin's latest moves, saying that "it's actions, not just words, that will be critical."

But the positive signals from Putin may delay a new package of sanctions on Russia's key economic sectors designed by the U.S. and its European allies, U.S. officials said.

Obama press secretary Josh Earnest

White House spokesman Josh Earnest said Putin will be judged on actions, not words, when it comes to his country's relations with Ukraine. (Jacquelyn Martin/AP)

‚ÄčPenalizing large swaths of the Russian economy, including its lucrative energy industry, would ratchet up the West's punishments against Moscow over its threatening moves in Ukraine. The U.S. and Europe have already sanctioned Russian individuals and entities, including some with close ties to Putin, but have so far stayed away from the broader penalties, in part because of concern from European countries that have close economic ties with Russia.

Samantha Power, the U.S. ambassador to the United Nations, demanded that Putin curb what she called "contradictory behaviour," including the return of a significant number of troops to the Ukrainian border.

"We have urged Russia to be part of the political solution to the crisis in Ukraine," Power said.