(Changes sourcing, adds quotes and FM comment)

MOSCOW, June 20 (Reuters) - Russia described Ukraine's ceasefire announcement on Friday as an ultimatum to pro-Russian separatists and said the shelling of one of its checkpoints on the border with Ukraine cast doubt on Kiev's peace plans.

A Russian customs officer was wounded in the shelling, according to Moscow's foreign ministry.

The Kremlin said in a statement it would carefully review a proposal by President Petro Poroshenko for peace in eastern Ukraine, where a separatist rebellion erupted in early April.

But it dismissed his declaration of a week-long ceasefire, saying it did not appear to include an offer of dialogue.

"This is not an invitation to peace and negotiations but an ultimatum to militias in the southeast of Ukraine to lay down their arms," the Kremlin said. "So far one major element is missing - a proposal to start negotiations."

It also said a Russian checkpoint on the border with Ukraine had come under "accidental or deliberate" mortar fire and demanded "an explanation and an apology" from Kiev.

In a separate statement, the foreign ministry said a Russian customs officer had been seriously injured when its border checkpoint at Novoshakhtinsk came under fire by Ukrainian forces.

It described the incident as a "direct provocation" that cast doubt on Kiev's declaration of a ceasefire.

"The Russian side expresses its strong protest against this aggressive act," the ministry said on its website. "We demand an urgent and swift investigation by Kiev." (Reporting by Alissa de Carbonnel; Editing by Tom Heneghan)

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