Europes role in ukraine crisis
US, G7 ready to impose more sanctions on Moscow
US Treasury Secretary Jack Lew met with Ukrainian leaders in Washington.
The compromise reached at an EU summit maintains EU unity on sanctions against Moscow.
Fighting in eastern Ukraine is at its worst level since a ceasefire was agreed last September.
Russia says weaponry from NATO states will abet war crimes in Ukraine and worsen the situation.
Armed rebels took positions late on Thursday around the airport controlled by Ukrainian forces.
The government kept up military pressure against pro-Russian rebels on Tuesday.
Three-way talks involving pro-Moscow rebels aim to pave the way for a new ceasefire.
Official said more than 300 rebel fighters had been killed and about 500 wounded.
The Ukrainian army rolled out an offensive against pro-Russia separatists holding the city of Slaviansk.
Kiev says Moscow must take concrete steps, including withdrawing its troops from the country.
The Russian keader has called on separatists to postpone the referendum planned for 11 May.
The SBU security service said one military Mi-24 helicopter gunship had been shot down, killing one.
Hundreds of separatists smashed their way into the provincial government headquarters in Luhansk.
Tensions have risen in the mainly Russian-speaking east since the overthrow of Ukraine's president.
Ukraine’s security said separatists occupying the security headquarters had planted bombs in the building.
Kerry says Washington still considers Russian actions in Crimea to be illegal and illegitimate.
Vladimir Putin aims to erect a Eurasian Union of former Soviet states
The US and EU agreed to work together to prepare possible tougher economic sanctions.
Barack Obama will seek support for his firm line at a meeting with other leaders of the G7.