* At least 5 reported dead in worst street violence since late January
* Security forces set 1600 GMT deadline to end disturbances
* Opposition want president to cede control of government
* Fresh clashes take place after Russia pledges more aid (Adds reported deaths)
By Pavel Polityuk and Richard Balmforth
KIEV, Feb 18 (Reuters) - Ukrainian riot police advanced on the heart of 12-week-old protests against President Viktor Yanukovich on Tuesday and security forces set a deadline to end disturbances after at least five protesters were reported killed in a day of clashes.
Forces loyal to the Russian-backed leader broke through front-line barricades near the Dynamo Kiev soccer stadium and marched to the edge of occupied Independence Square hours after Moscow gave Ukraine $2 billion in aid it had been holding back to demand decisive action to crush the pro-European protests.
Clashes raged for several hours outside the parliament building, where opposition lawmaker Lesya Orobets said three demonstrators were killed and taken to a nearby officers' club used as a medical centre. More than 100 people were injured, she said.
"Three bodies of our supporters are in the building. Another seven are close to dying (because of wounds)," she said on her Facebook page.
Two more bodies were lying in front of a Metro station on the southeastern side of Independence Square, a photographer told Reuters.
The casualty reports could not be independently confirmed.
The State Security Service (SBU), in a joint statement with the interior ministry, set protesters a 6 p.m. (1600 GMT) deadline to end street disorder or face "tough measures".
"If by 6 p.m. the disturbances have not ended, we will be obliged to restore order by all means envisaged by law," the statement said.
The defence ministry issued a separate warning to protesters to evacuate the officers' club near parliament.
However, it was not clear whether the ultimatums presaged an imminent assault on the core opposition encampment on Independence Square, known as Maidan, where peaceful demonstrators have held a protest vigil for three months.
Nationwide protests against Yanukovich erupted in November after he bowed to Russian pressure and pulled out of a planned far-reaching trade agreement with the European Union, deciding instead to accept a Kremlin bailout for the former Soviet republic's heavily indebted economy.
Monday's $2 billion cash injection, a resumption of the $15 billion aid package, was seen as a signal that Russia believes Yanukovich has a plan to end the protests and has dropped any idea of bringing opposition leaders into government.
In another apparent gesture towards Moscow, a Ukrainian government source said state gas company Naftogaz has paid back $1.3 billion of its 2013 debt to Russian gas monopoly Gazprom , although it still owes $1.5 billion.
Ukraine's hryvnia currency fell towards five-year lows after the fresh outbreak of violence, with importers seeking dollars.
As protesters and police battled on the streets of Kiev, Russia called the escalation a "direct result of connivance by Western politicians and European structures that have shut their eyes ... to the aggressive actions of radical forces".
While Russian President Vladimir Putin seems to have won the battle for influence in Ukraine for now, protesters who have occupied the centre of the capital are not going quietly.
"I think Russia received some kind of assurances from the Kiev leadership that were satisfactory, because only a day before there was nothing like it," said Gleb Pavlovsky, former Kremlin adviser and political analyst in Moscow.
"I think Yanukovich showed he would stick firmly by his position in talks (with the opposition), he would not make excessive concessions, he would fight the radicals who are getting stronger in the opposition ... and that the (new) prime minister would not be a member of the opposition."
But rather than boosting Yanukovich, Moscow's move may have helped to provoke a more violent turn in the protests, especially from those demonstrators who have a strong anti-Kremlin agenda.
Several thousand protesters torched vehicles and hurled stones in the worst violence to rock the capital Kiev in more than three weeks.
Police replied by firing rubber bullets and stun and smoke grenades from trucks and from the tops of buildings, forcing the protesters back by about 100 metres.
"The authorities do not want to compromise on any issue ... We understand that yet another odious candidate will be put forward (for prime minister), one who will be unable to restore the economy or end the political crisis," said Vyacheslav Kyrylenko, an opposition deputy.
Right Sector, a militant far-right group, added to tension by calling on people holding weapons to go to Independence Square, centre of the revolt, to protect it from a possible move by security forces to break it up.
Inside parliament, where opposition leaders brought proceedings to a halt by blocking the speaker's tribune, opposition leader Vitaly Klitschko urged Yanukovich to take riot police off the streets to avert further "conflict in society".
"It will be the decision of a real man," the boxer-turned-politician told reporters inside parliament.
The protesters had marched to the parliament building to press the opposition leaders' calls for Yanukovich to relinquish what they call his "dictatorial" powers and particularly his control of the economy and the security forces.
But when they were blocked by a line of trucks about 100 metres from the building, they hurled stones at police, a Reuters witness said, and set three trucks ablaze with petrol bombs.
As the clashes extended into early afternoon, protesters ransacked a nearby office of Yanukovich's Party of the Regions.
"WE'RE NOT FOR SALE"
In what has become a geo-political tussle redolent of the Cold War, the United States and its Western allies are urging Yanukovich to turn back to Europe and the prospect of an IMF-supported recovery, while Russia accuses them of meddling.
A senior EU official in Brussels said aid from the European Union and/or The International Monetary Fund was still available, though it is tied to reforms.
"It must be done in the context of a Ukrainian government really eager to reform and to do the necessary reform for its economy, in the energy sector in many other sectors, in the modernisation of industry etc," the official, who asked not to be named, said.
News of the fresh credit from Russia failed to cheer the currency market, where the Ukrainian hryvnia fell by up to 1.6 percent against the dollar on Tuesday, Reuters trading system showed.
It also failed to impress the protesters. "We don't need this money from Russia because it is not meant to help but to buy us. But we are not for sale. Can't they see that this is simply a dirty bribe?," said protestor Valentin Sypko. (additional reporting by Elizabeth Piper in Moscow, writing by Richard Balmforth, editing by Jeremy Gaunt)