Ukraine’s new president has declared a week-long unilateral ceasefire and unveiled a sweeping plan for curbing a pro-Russian insurgency that has killed 13 more soldiers in fierce clashes in the eastern rustbelt.

“Today, on June 20, the ceasefire should begin. It will last through June 27,” Petro Poroshenko told local residents on Friday in a small village that served as a popular lakeside resort before Kiev unleashed a full-scale “anti-terror” campaign on April 13.

The military operation’s official spokesman said Ukrainian forces would formally halt fire at 10pm.

But a senior rebel commander immediately rejected Poroshenko’s plan for ending unceasing battles that have killed more than 375 people and left the ex-Soviet country on the verge of splitting in two.

“No one will lay down their arms until a full troop withdrawal from our land,” said Valeriy Bolotov of the self-declared Lugansk People’s Republic.

A Ukranian national security spokesman reported that the latest battles had claimed the lives of 13 soldiers – a toll underscoring the uptick in violence in the past week.

The sense tensions on the ground were rising was reinforced when Washington followed up similar charges from NATO by accusing the Kremlin of stirring up new trouble along its neighbour’s western frontier.

A senior US administration official said Russia had deployed “significant” military forces near Ukraine “to provide active support for separatist fighters”.

The US official said the troops were “the closest they have been since the invasion of Crimea” in March.

Russian President Vladimir Putin’s spokesman had earlier confirmed deploying some units to “reinforce the protection of the Russian border”.

Poroshenko’s ambitious push for peace followed two calls he had placed to Putin within 72 hours in the belief no truce would work without the Russian strongman’s support.

The 48-year-old chocolate baron on his first visit to the restive industrial region of Donetsk since his June 7 inauguration said seven days provided the rebels “more than enough time” to show they were willing to engage in direct talks.

Poroshenko’s announcement automatically enacted a 14-point plan demanding the rebels disarm and promising to decentralise power through constitutional reform.

The initiative drops criminal charges against fighters who committed no “serious crimes” and provides “a guaranteed corridor for Russian and Ukrainian mercenaries to leave” the conflict zone.

And it establishes a 10-kilometre border buffer zone to stem the flow of gunmen and military equipment both Kiev and Washington claim have been flooding in from Russia in recent weeks.

But it also calls on “local government bodies to resume their operations” – a demand rejected by separatist leaders who have proclaimed their independence from Kiev and occupied administration buildings in about a dozen cities and towns in the east.

The plan was the product of 10 days of European-mediated talks in Kiev between a Poroshenko envoy and Russia’s ambassador to Ukraine.