A clutch of rusty old naval boats bob about mournfully in the Black Sea off the harbor of Ukraine's southern city Odessa near an ageing missile boat.

"The only way to get those pieces of junk moving is to get some paddles and start rowing," an officer says derisively.

NATO on Wednesday expressed alarm over a "wide-ranging" military build-up by Russia in Crimea, as well as talk that Moscow could even install nuclear weapons on the annexed Black Sea peninsula.

A statement issued during two days of talks by the military alliance's top diplomats, in the Turkish resort of Antalya, said "we do not and will not" recognize Russia's 2014 annexation of Crimea from Ukraine.

U.S. Secretary of State John Kerry on Wednesday warned Russia that now was a critical time to implement a fragile peace deal to end the fighting in Ukraine, after his talks with President Vladimir Putin raised hopes of a slackening in tensions.

Kerry said there was an "enormous moment of opportunity" to bring to an end over a year of fighting in eastern Ukraine between government forces and pro-Russia separatists which has dragged relations between Moscow and the West to a new post-Cold War low.

Russia vowed Tuesday it was ready to work with the United States on a host of burning issues but insisted it would not bow to "coercion," as President Vladimir Putin hosted top U.S. diplomat John Kerry. 

On the highest-level U.S. visit to Russia since the conflict in Ukraine erupted in late 2013, Putin met with Kerry at his summer residence in the Black Sea resort of Sochi.

Three Ukrainian soldiers have been killed and another one injured as clashes with pro-Russian rebel fighters flared despite a shaky truce, Kiev's military spokesman said Tuesday.

"The situation is still tense" in the conflict zone, spokesman Andriy Lysenko told journalists in Kiev. "Over the past 24 hours three Ukrainian soldiers died and one was injured in fighting."

Allies of murdered Russian opposition leader Boris Nemtsov on Tuesday unveiled a report he worked on claiming that hundreds of Russian soldiers have been killed fighting in Ukraine.

The report -- based on media articles and information from families of Russian soldiers allegedly killed in Ukraine -- repeats widespread allegations of Russian military involvement in Ukraine that Moscow has consistently denied.

Russia and pro-Moscow rebels in eastern Ukraine now have the capacity to launch new attacks "with very little warning" after a sustained military build-up, NATO head Jens Stoltenberg said Monday.

"In eastern Ukraine, Russia and the separatists (now) have capabilities which enable them to launch new attacks with very little warning time", violating a ceasefire agreed in February, Stoltenberg said.

German Chancellor Angela Merkel on Sunday said there was still no genuine ceasefire in eastern Ukraine, as she paid a visit to Moscow to commemorate millions of Soviet soldiers and civilians who died in WWII.

"Unfortunately we still do not have a ceasefire today," Merkel said after talks with Russian President Vladimir Putin. 

German Chancellor Angela Merkel on Sunday paid tribute to Soviet soldiers killed in WWII as she called for cooperation with Russia amid tensions over Ukraine.

Merkel flew to Moscow to lay a wreath at the grave of the Unknown Soldier near the Kremlin walls, in an apparent compromise gesture after she skipped Russia's main festivities on Saturday.

Ukraine's pro-Russian rebel leader Alexander Zakharchenko said Saturday that insurgents had released two US aid workers detained for alleged spying. 

"We have freed two Americans that we arrested earlier. One of the two is a CIA agent, the other was enlisted," Zakharchenko told journalists, without giving any more details.