Ukraine witnessed the deadliest day on Thursday in almost three months of anti-government demonstrations, as more people were killed after violent street clashes between protesters and riot police erupted anew despite a truce announced by the country's president.

Government snipers fired at protesters who advanced on police lines in central Kiev, killing at least 70 people and wounding hundreds of others, according to opposition medics.

Meanwhile, the Kiev City Administration, without giving a breakdown, said 67 people have been killed in three days of violence since Tuesday. There was no way to immediately verify any of the death tolls.

Moreover, Interior Minister Vitaly Zakharchenko said at least 13 police officers had been killed and 67 others captured by radical demonstrators.

He also said police had been issued with combat weapons and would use them "in accordance with the law" to defend themselves.

"As part of the work of the anti-terrorist centre of the Security Service of Ukraine, I have signed the respective orders – the law enforcers have been handed combat weapons," Zakharchenko said in a statement.

Anti-government demonstrators have used assault weapons against police, he said, urging radical activists to voluntarily surrender their arms and return to peaceful protests.

Meanwhile, Ukraine's ombudsman for human rights Valeria Lutkovska said the international standards of law enforcement agencies do not allow the use of deadly force to suppress riots.

"There are no international standards which allow law enforcement agencies to shoot to kill to stop the riots. Contrary, international instruments call for minimising damage from the use of force," Lutkovska said in a statement.

Thursday's fresh violence came several hours after President Viktor Yanukovych announced a truce with protesters following the death of 26 people in violent street clashes, which incurred the threat of sanctions from the European Union (EU).

Also on Thursday, the foreign ministers of Germany, France and Poland met Yanukovych and three main opposition leaders in Kiev, trying to broker a compromise solution between the opposing sides.

Olha Lappo, a spokesperson for one of the opposition leaders, said the meeting had discussed the steps designed to end the bloodshed and to avoid further escalation of the conflict.

The ministers have decided to extend their stay to Friday to continue negotiations and they would miss a special summit on the Ukrainian crisis in Brussels.

Protests in Ukraine began in November when Yanukovych shelved a trade and economic agreement with the EU and turned instead to Russia for financial aid. The rallies turned violent in mid-January, when at least four people were killed in clashes between riot police and activists.

The conflict escalated on Tuesday, when protesters attacked police with Molotov cocktails and set fires outside parliament. Police responded with stun grenades and water cannons.