UN political chief Jeffrey Feltman told an emergency UN Security Council meeting on Friday that the cease-fire in Ukraine would have to be fully respected and the protection of civilians given priority.

"We are either looking at a return to a deepening intractable conflict or a momentary upsurge in parts of the conflict zone," he told the council, adding, "We cannot afford either scenario."

The statement comes as the last round of talks under the initiative of the Organization for Security and Cooperation in Europe (OSCE) had ended without visible results earlier this week.

"We urgently call on the sides to return to the negotiations table and resolve all the questions," OSCE monitoring mission deputy head Alexander Hug told the United Nations Security Council.

"The violence witnessed in and around the town of Marinka constitutes a new, worrying development in the conflict."

Ukraine crisis on G7 agenda

The Ukraine crisis will also be high on the agenda of the upcoming G7 meeting in Germany - without Russia allowed to attend. Ukrainian President Poroshenko said that he would speak by phone with US President Barack Obama and German Chancellor Angela Merkel ahead of the gathering.

Germany's foreign minister Frank-Walter Steinmeier had also voiced his worries about an escalation of the crisis ahead of the summit.

The United States, the EU and Germany have all expressed alarm over the renewed violence, with Washington warning Moscow that it might face new sanctions if the crisis escalated any further, adding that the US might get actively involved in the conflict.

Deadly clashes in Marinka

Street fights in eastern Ukraine have killed nearly 30 people including over the past few days. Clashes erupted near the settlement of Marinka, just west of Donetsk, with both sides reporting fresh instances of shelling since the weekend and particularly heavy fighting on Wednesday.

Ukraine has accused pro-Moscow rebels of revving up their offensive, while separatist forces claim that the Ukrainian military is to blame for the fighting.

There were also reports of rebels firing on a village in the region of Lugansk and another village near the government-controlled city of Mariupol, the Ukrainian military said.

Poroshenko warns of 'colossal threat' to stability

Ukrainian President Petro Poroshenko warned of a threat of a full-blown Russian invasion in the east of the country, adding that Moscow had deployed "an unprecedented" number of troops along the border.

"There has never been such an amount of troops near our borders since August," Poroshenko said, referring to the height of the hostilities last year, during which Kyiv regular Russian troops were involved in addition to rebel forces.

"I believe that there must be a reason to have such a powerful force in place. The threat of Russian invasion is unprecedented. The entire Russian military arsenal is already there, everything but submarines."

In an address to parliament on Wednesday, Poroshenko warned of a "colossal threat" of a serious escalation, claiming that more than 9,000 Russian soldiers were on Ukrainian soil to support the insurgency. He used the upsurge in Russian military presence to justify his mobilization of 50,000 Ukrainian troops in the crisis region. The president also repeated his plea to the UN to send peace-keeping troops.

However, the Ukrainian also army admitted that it had resorted to the use of heavy artillery banned by the European-brokered peace agreement, and accused the rebels of using weapons also prohibited by the truce deal.

According to UN figures, more than 6,400 people have been killed in the fighting since April 2014.

sbs/kms (AFP, dpa)