Western-backed President Petro Poroshenko Thursday named a new defence minister and top general as his forces reported pressing on with an offensive to oust pro-Russian insurgents from the restive country's industrial east.

New Defence Minister Valeriy Geletey immediately pledged to deliver a knockout blow to the rebels after Poroshenko this week ditched a 10-day truce.

"I am sure that Ukraine will win and believe me that there will be a victory parade," the 46-year-old former police commander told lawmakers.

Ukrainian troops backed by tanks and bomber jets have stepped up their campaign since a shaky ceasefire failed to stem 11 weeks of violence that have claimed more than 460 lives.

But the poorly trained and ill-equipped force is being heavily tested by highly organised militias both Kiev and the West accuse the Kremlin of arming and funding — a charge Russian President Vladimir Putin flatly denies.

The border service said that nine guards were wounded on Thursday when shells hit a key crossing with Russia that Kiev had celebrated retaking just days earlier.

Gunmen dressed in camouflage also shot dead three Ukrainian policemen and wounded another in the rebel-held city of Donetsk.

Regional authorities said at least one resident of the flashpoint city of Kramatorsk was killed in unceasing rebels raids that forced workers to ask the local heavy equipment manufacturer to suspend its operations.

- Obama, Merkel press Putin -

The latest battles came despite a flurry of diplomacy aimed at convincing the warring sides to agree to a fresh round of indirect talks to hammer out a new ceasefire.

French President Francois Hollande and German Chancellor Angela Merkel asked Putin by phone to help organise another meeting of the so-called Contact Group by the weekend.

Merkel's office said she and US President Barack Obama also spoke on the phone and urged the Russian leader of "the importance of a rapid ceasefire observed by both sides".

"Russia must first contribute by ensuring that the separatists in eastern Ukraine uphold the ceasefire," the German account of the call said.

The White House said Obama and Merkel agreed that if Russia did not de-escalate the situation in eastern Ukraine in "short order," Washington and Europe would take further "coordinated" steps towards sanctions.

Putin has thus far refused to demand that the rebels halt fire. He also adamantly blames Poroshenko for the failure of the original truce.

Both Kiev and the rebel command refuse to negotiate directly. The West would now like to arrange "consultations" in which Kiev is represented by former president Leonid Kuchma.

Poroshenko told US Vice President Joe Biden on Thursday that he was "ready to return to the old ceasefire" under two conditions.

The first required the fighters to firmly commit themselves not to stage attacks against government roadblocks that were prevalent during the earlier truce.

The second concerned "the release of all hostages and the establishment of border controls that include OSCE monitors".

Moscow has insisted that the talks be convened before any new border agreement is reached.

Where exactly such a meeting could be held is also unclear. Previous discussions have taken place in Donetsk but Kiev now appears reluctant to meet there again.

- 'Explosion of violence' -

German Foreign Minister Frank-Walter Steinmeier has warned that Ukraine faces "an explosion of violence" if a new ceasefire fails to emerge.

The international call for talks is a point of rare agreement between Russia and the West in their Cold War-style battle for the future of the strategic ex-Soviet state.

Putin accuses Kiev of waging a "punitive operation" — the term Moscow once used to portray Nazi atrocities during World War II — while the West backs Ukraine's right to fend off Russian-backed "aggression".

Washington and the European Union are also threatening devastating economic sanctions that could cut off Russia's entire banking system from the West and restrict its important arms exports.

The EU "has made clear that we are ready to impose further sanctions on Russia if they fail to take the necessary steps to end military hostility", British Foreign Secretary William Hague told reporters after phone talks with his Ukrainian counterpart.

An EU diplomat said this week that the 28-nation bloc — still not ready to endanger strong financial and energy ties with Russia — was preparing new punitive steps that fell short of the "sectoral" sanctions promoted by the United States.