An elderly woman walks to a bus stop in Donetsk's Kievsky district on Thursday. The neighborhood, close to the city's airport, has seen heavy shelling over the past months. Photo: AFP

 The EU is ready to step up sanctions on Russia over the Ukraine conflict, but its priority is to bolster a fragile cease-fire agreed in Minsk, EU foreign policy chief Federica Mogherini said on Friday.

Mogherini, attending a meeting of EU foreign ministers in Latvia, said the need now was to strengthen ceasefire monitors from the Organization for Security and Co-operation in Europe (OSCE), a rights and security watchdog.

Despite some signs of improvement in eastern Ukraine since a new cease-fire was agreed last month, there was no question of the EU easing sanctions now, she said.

"They will not be lifted until something really good happens on the ground and on the other side we are always ready to increase the pressure if needed," Mogherini told a conference.

The EU has joined the US in imposing sanctions on Russia but has found it increasingly difficult to agree on tougher measures as many member states rely on Russia for energy.

Meanwhile, the second-highest ranking US diplomat promised on Friday to maintain pressure on Russia via economic sanctions to get it to change tack on Ukraine but appeared to rule out arming Kiev, saying Moscow would easily be able to send many more weapons. US President Barack Obama and European leaders are weighing their next steps to try to halt a conflict that has killed about 6,000 people since last April.

US and European officials are concerned that sending arms to Kiev would risk escalating the fighting and suck them into a proxy war with Russia. But some US lawmakers are urging Obama to counter what they see as increased aggression by Russia by providing weapons to Kiev.

"We must keep up the pressure we are exerting to persuade Russia to change the direction it has struck out on in Ukraine," US Deputy Secretary of State Antony Blinken told German radio DLF in an interview broadcast on Friday.

Blinken said Ukraine "needs the ability to defend itself" and the US is already providing non-lethal military support, he said.

But, asked about the possibility of the US sending weapons, he said he did not believe there could be a military solution. "We also know that if we were to supply more defense technology, Russia could easily counter that by providing two, three or four times what we do," said Blinken whose comments were translated from English to German.

Moscow denies Western and Ukrainian accusations that it is backing the rebels in eastern Ukraine with troops and weapons.

Russian officials on Thursday also dismissed a claim by the US that Moscow has sent "thousands" of troops to fight alongside pro-Russian separatists in Ukraine.

"These figures, which are plucked out of the air, of course demoralize and disorientate the international community," foreign ministry spokesman Alexander Lukashevich said.

US Assistant Secretary of State Victoria Nuland on Wednesday told a congressional foreign affairs committee that Russia had deployed "thousands and thousands" of troops to Ukraine, although she said she could not give a precise number. She also accused Russia of sending hundreds of pieces of military equipment to the rebel-controlled zone since December, including tanks and rocket systems. Russia's deputy defense minister also denied the allegation.