The Ukrainian military said in a statement released to reporters on Thursday that four of its soldiers had been killed and 18 others wounded over the past 24 hours. The statement said much of the fighting had been centered around Donetsk airport.

Pro-Russia rebels said three people had been injured in shelling in Donetsk overnight.

This comes a day after the Western military alliance NATO accused Russia of sending large numbers of troops and heavy weapons over the border into Ukraine.

US General Philip Breedlove, NATO's commander in Europe, said during a visit to Sofia on Wednesday that "columns of Russian equipment, primarily Russian tanks, Russian artillery, Russian air defense systems and Russian combat troops" had entered Ukraine.

Observers from the Organization for Security and Cooperation in Europe have also reported seeing a number of military convoys headed to rebel-held regions in recent days.

UN Security Council meets

Later, the United Nations Security Council held another emergency session on the crisis in Ukraine, but did not decide on any concrete action.

The UN's under secretary-general for political affairs, Jens Toyberg-Frandzen, told the Council that the UN was "deeply concerned over the possibility of a return to full-scale fighting" or that the conflict could become "frozen" for a period of years or decades.

Washington's ambassador to the UN, Samantha Power, said Moscow "talks peace but it keeps fueling war" in eastern Ukraine. She also accused the Kremlin of seeking to undermine a ceasefire agreed in the Belarusian capital, Minsk, back in September, which, despite frequent breaches, had until recently kept a lid on major fighting.

Russia's deputy ambassador to the UN, Alexander Pankin, rejected NATO's that his country was sending troops and military hardware into eastern Ukraine as a "foray into propaganda."

An estimated 4,000 people have been killed since fighting broke out between pro-Russia separatists and government troops in eastern Ukraine back in April.

Russian warships off Australia

Meanwhile the Royal Australian Navy has said it is tracking four Russian military vessels, including a "heavily armed" cruiser and destroyer in international waters off its northern coast. This comes ahead of the G-20 Summit, to be held in Brisbane on the weekend, which Russian President Vladimir Putin is to attend.

Speaking at the East Asia Summit in Myanmar's capital, Naypyidaw on Thursday, Australian Prime Minister Tony Abbott described the deployment of the Russian warships as a sign of the Kremlin's "assertiveness" but added that it was also "not unusual" ahead of a major conference.

pfd/rc (AFP, AP, dpa, Reuters)