A spokesman for Ukraine's Security Council said the army was not responsible for any explosions on Monday in the centre of the eastern city of Donetsk, where rebels earlier reported Ukrainian tanks trying to cross the city limits.
"There is work on clearing approaches to the city, on destroying checkpoints of the terrorists. If there are explosions in the middle of the city - then it is not Ukrainian soldiers," Andriy Lysenko told a news conference.
"We have strict orders not to use air strikes and artillery in the city. If there is fighting in the city, we have information that there is a small self-organised group who are fighting with the terrorists."
Fighting raged near the railway station in the rebel-held city of Donetsk in eastern Ukraine on Monday, and witnesses saw plumes of smoke and heard loud explosions from the area around the station in the city centre.
A pro-Russian separatist leader said Ukrainian forces were trying to break into Donetsk and a Reuters witness saw two rebel tanks heading towards the railway station, driving against a steady flow of people running away.
A spokesman for Ukraine's military operations in eastern Ukraine said the operation was in an "active phase" but could not comment on reports of troops entering Donetsk because he did not want to give away the Ukrainian strategy.
Also on Monday, two experts from the German air accident investigations authority (BFU) have arrived in Kiev and are trying to make their way to the crash site of the downed Malaysian passenger plane, a spokesman for the authority said.
Four Germans were amongst the 298 people killed when Malaysia Airlines flight MH17 was shot down on Thursday last week.
The director of the BFU, Ulf Kramer, and a further expert, who cannot be named for security reasons, arrived in Kiev late on Sunday and are trying to reach the crash site with other international investigators, the spokesman said.
Three members of a Dutch Disaster Victims Identification team had also arrived in the eastern Ukrainian city of Donetsk, the first international investigators to reach rebel-held territory since the airliner crashed last week.
This comes after rescuers found 251 bodies and 86 fragments of bodies by late on Sunday at the crash site of a Malaysian airliner and a second train with refrigerator wagons to take the remains away has arrived, a Ukrainian government committee said.
In a statement, the committee investigating the disaster said the first train where the remains of almost 200 victims have been placed before starting their journey home was stuck in the station in the town of Torez because "terrorists are blocking its exit".
Local reactions:
President Vladimir Putin should understand that "it's enough already", Ukrainian Prime Minister Arseny Yatseniuk said on Monday, criticising the Russian leader for handing weapons to rebels fighting Kiev's forces in eastern Ukraine.
"I do not expect anything from the Russian government. They supplied weapons, they sent in fighters. Putin should understand that it's enough already. This is not a conflict between Ukraine and Russia. It is an international conflict," he told a news conference.
Ukraine's prime minister also said Ukraine is willing to hand over coordination of an investigation into the Malaysian airliner crash to international partners but Kiev is convinced the plane was shot down by "professionals".
"Ukraine is ready to hand over a coordinating role in the investigation into the tragedy to our Western partners. And the Netherlands could head that process," Arseny Yatseniuk told a news conference.
"At the moment, we have no doubt that the plane was shot down. The reason for it - a missile strike most likely from a BUK-M1 (SA-11 radar guided missile system). It is clear that this system could not be operated by drunk pro-Russian terrorists. There were professional people," he said, referring to Kiev's charge Moscow had a clear role in shooting down the plane.
Russia may have to face stronger European Union sanctions after the downing of a Malaysian jet that killed 298 people, British Deputy Prime Minister Nick Clegg said on Monday.
"We do believe there is a case to move to what is called, in the jargon, 'tier three sanctions'... They do involve a wider range of individuals, of assets and of sectors," Clegg told reporters.
Britain said that it had agreed with Germany and France that the European Union should be ready to impose further sanctions on Russia when European foreign ministers meet in Brussels on Tuesday.
For his part, the Dutch prime minister said "all political, economic and financial options" were on the table if access did not improve to the area in eastern Ukraine where a Malaysian airliner went down with the loss of all 298 aboard.
Mark Rutte told parliament on Monday his government's priority was to recover and identify the bodies of the passengers, 193 of whom were Dutch.
"It is clear that Russia must use her influence on the separatists to improve the situation on the ground," Rutte said.
"If in the coming days access to the disaster area remains inadequate, then all political, economic and financial options are on the table against those who are directly or indirectly responsible for that," he said.
In turn, Russian President Vladimir Putin said the downing of a Malaysian airliner in east Ukraine must not be used for political ends and urged separatists to allow international experts access to the crash site.
He reiterated his belief that the incident would not have happened if Ukrainian government forces had not ended a truce and resumed a military campaign against the pro-Russian separatists who have risen up in eastern Ukraine.
REUTERS