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

Ms 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 ceasefire 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," Ms Mogherini told a conference.

"So far the ceasefire has been starting, not perfect with some violations still, but for sure the trend is a positive one," she added. "We need to strengthen the OSCE mission that is on the ground now."

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 heavily on Russia for energy.

The truce has eased pressure for new sanctions, but the EU must decide soon whether to extend economic curbs on Russia that expire in July.

Lithuanian Foreign Minister Linas Linkevicius, an EU hawk on Russia, said extending sanctions to the end of the year was "the least we can do".

"Everyone is for unity but you know unity to do nothing is not for me. I don't like it. So we have to be united on doing something," he said.

Violence is continuing in eastern Ukraine despite the ceasefire accord, which calls for the withdrawal of heavy weapons.

Russia denies accusations by Ukraine and Western governments that it has been sending troops and weapons to support separatists.

Ms Mogherini warned against a return to a Cold War mentality.

"Any attempt to go back to a logic of confrontation that recalls the 50s is not a European logic. And if someone else in Moscow falls into that trap or promotes that trap it is not for us to follow that mistake."

Foreign Secretary Philip Hammond said in Warsaw the EU would prepare possible new sanctions on Russia that could be imposed quickly if the Minsk deal was broken.

Meanwhile, the second-highest ranking US diplomat has warned that pressure to arm Ukraine would increase if aggression from pro-Russian separatists persisted, even though Moscow would easily be able to send more weapons into the country than the West.

Deputy Secretary of State Antony Blinken said the US was already working on supplying Kiev with non-lethal equipment, including protective vests, night-vision goggles and other equipment.

"But if the aggression continues, I think there will be more and more pressure to give them other means to protect themselves," he said.

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 President Barack Obama to counter what they see as increased aggression by Russia by providing weapons to Kiev.

Mr Blinken said he did not believe there could be a military solution and the emphasis must be on diplomatic efforts.

He added: "And you know, it's certainly true that, were any of us to provide weapons to Ukraine, Russia could match that and then double that and triple that and quadruple that.

"But it's also important to have in mind that the Ukrainians should be able to defend themselves and we provided significant security assistance, defensive, non-lethal security assistance to Ukraine to do just that."

Accusing the separatists "who are armed and supplied by the Russians" of repeatedly breaking ceasefires declared by Ukraine throughout the conflict, Mr Blinken said: "At some point you have to say almost as a moral proposition, we have to do whatever we can to help them defend themselves against aggression."