The Ukrainian opposition vowed to press on with protests Sunday despite an offer of top posts made by President Viktor Yanukovych, as protesters seized another official building in the capital.

Thousands of protesters, some clutching baseball bats and wearing gas masks, were again starting to fill the center of Kiev for new protests as tensions showed no sign of ending in the deadly crisis gripping the country.

Ukrainian President Viktor Yanukovych on Saturday offered the opposition the post of prime minister and to change the constitution, in a proposed compromise deal aiming to end the country's worst post-independence crisis.

Yanukovych offered top opposition leaders Arseniy Yatsenyuk and Vitali Klitschko the posts of prime minister and deputy prime minister in a brand new government, the presidency said.

A Ukrainian anti-government protester found brutally murdered in a forest outside Kiev was on Friday given a hero's burial in his western home city of Lviv, in an emotional ceremony attended by thousands of mourners.

Yuriy Verbytskiy was abducted this week in Kiev by a group of individuals who dumped his body in a forest outside the city.

Building up barricades several meters high guarded by activists in body armor, Ukrainian protesters have turned their Kiev protest camp into a fortress they believe will be impossible to conquer without the greatest force.

In a scene recalling the fortified towns of the Middle Ages, protesters have built the barricades to protect their self-contained protest zone centered around the hub of Independence Square in central Kiev.

The United States is in close contact with European allies to try to end the violence in Ukraine, Secretary of State John Kerry said Friday.

"We will stand with the people of Ukraine," Kerry vowed at the World Economic Forum meeting in the Swiss town of Davos.

Lithuania said Friday it will provide free medical assistance to Ukrainians injured in violent protests in Kiev in a show of support for the pro-EU opposition.

The first group will include around 10 wounded protesters and will arrive in the Baltic state on Monday for treatment and physiotherapy, Lithuanian presidential adviser Jovita Neliupsiene told Agence France Presse.

A prominent Ukrainian opposition activist has been reported missing in Kiev following the apparent abductions of two other opponents of President Viktor Yanukovych, one of whom was found dead.

Dmytro Bulatov is a 35-year-old activist from the Avtomaidan group, car drivers who hold motorcade protests against Yanukovych. He was reported missing by his wife late Thursday, Kiev police said.

Arnold Schwarzenegger on Friday sent a video message of support to anti-government protesters in Ukraine, wishing them luck and making a peace sign.

"I want to send a message to the people of the Ukraine to let them know that I wish them all the best of luck in their peaceful struggle for democracy and freedom," said the deeply tanned action star and former California governor.

France and Germany on Friday summoned the Ukrainian ambassador over Kiev's response to five days of pro-EU protests in which activists say five people have died in clashes with security forces.

"We want to show that we are serious with our criticism," German foreign ministry spokesman Martin Schaefer told reporters, after the French government made the same move.

President Viktor Yanukovych on Friday pledged a reshuffle of the government next week to ease Ukraine's crisis but also threatened to use "all legal methods" if a solution was not found.

When parliament meets in an extraordinary session next Tuesday "we will take a decision about reshuffling the government," he said at a meeting of religious leaders, vowing that parliament would also discuss changes to controversial anti-protest laws.