Fighting around two flashpoint cities in eastern Ukraine on Sunday rattled a tenuous truce between government troops and pro-Russian rebels less than 48 hours after it came into force.

Insurgent militias bombarded a government-held checkpoint on the eastern edge of the port city of Mariupol overnight, local officials said, killing one woman and triggering panic among residents.

U.S. Secretary of Defense Chuck Hagel pledged on Sunday to boost military ties with Georgia amid the unfolding crisis in Ukraine.

"Our shared goal is to build even stronger military ties in the future, particularly in the light of Russia's blatant aggression in Ukraine," Hagel told a press conference in Tbilisi.

Amnesty International accused rival fighters in the Ukraine conflict of war crimes including indiscriminate shelling, abductions, torture, and killings, in a report compiled ahead of the ceasefire deal.

In flashpoint cities of eastern Ukraine, uneasy residents voiced doubt Saturday that the ceasefire deal would bring lasting peace after five months of devastating conflict that has ruined the lives of so many.

"Nothing has actually changed," 38-year-old Vladyslav Lobsin told Agence France-Presse in the strategic port city of Mariupol, where residents had been bracing for a rebel onslaught until the guns were ordered silent.

Ukraine said Saturday a truce was largely holding in the war-battered east, despite fears it may ultimately fail to halt a pro-Russian insurgency still threatening to tear the country apart.

The 12-point pact signed on Friday is the first backed by both Kiev and Moscow to end a conflict which triggered the most serious crisis between Russia and the West since the Cold War.

An intensified underwater search for missing Malaysia Airlines Flight MH370 will start in about two weeks' time, Australian premier Tony Abbott said Saturday as he visited Malaysia to discuss the issue.

Abbott said the hunt for the jet, which inexplicably veered off its Kuala Lumpur-Beijing route on March 8 with 239 people aboard, would continue for as long as necessary.

NATO leaders agreed Friday to set up a rapid reaction force as part of efforts to reassure allies rattled by the Ukraine crisis and rising Islamic extremism, alliance head Anders Fogh Rasmussen said.

As a result, NATO will be able to "maintain a continuous presence... in eastern parts of the alliance on a rotational basis," Rasmussen said, denouncing Russian aggression in Ukraine.

As the thunder of heavy combat echoed nearby, 25-year-old Yelena said she and her friends were ready to up sticks and flee the Ukrainian port of Mariupol, fearful of a major rebel assault on the city despite hopes of a truce.

"We don't believe (there will be a ceasefire) but we can hope," she told AFP on the eastern edge of the flashpoint city where fresh fighting erupted shortly before talks on the proposed truce open in Belarus.

Ukraine's Russian-backed separatists said they had agreed with Ukrainian officials on a ceasefire starting Friday, at peace talks in Minsk.

"Representatives of Ukraine and Donetsk People's Republic and Lugansk People's Republic signed a ceasefire protocol from 6 pm on Friday," the Twitter account of the so-called Donetsk People's Republic said, without providing further details.

The United States is preparing fresh sanctions against Russia over the Ukraine crisis in coordination with Brussels, which plans to announce further punitive measures on Friday, a White House official said.

"We work on this in coordination with the Europeans," Deputy National Security Adviser Ben Rhodes said on Thursday, adding: "If Russia escalates (in Ukraine), we are prepared to escalate our pressure."