The Ukrainian situation remains "highly precarious", while the new Ukrainian government faces "massive" political and economic challenges, British Prime Minister David Cameron said Thursday.

Speaking at an EU leaders' meeting in Brussels, Cameron said "the slightest miscalculation could see (the Ukrainian situation) spiral out of control."

"We are facing the most serious crisis in Europe this century," he reiterated.

He said Britain, along with other EU countries, is prepared to offer "a powerful package of finance, trade and technical assistance" to Ukraine, on condition that the interim Ukrainian government carries out needed reforms and tackles corruption.

"The new Ukrainian government faces massive challenges. They will need to carry out far-reaching reform needed to stabilize and repair their economy," he said.

Cameron said EU leaders have tasked the European Commission to start work on "additional measures", such as travel bans and asset freezes, as possible "consequences" for Russia.

However, he warned that getting agreement from leaders of 28 European nations is "never easy."

EU leaders on Thursday decided to suspend negotiations on a more liberal visa regime for Russia, stop work on a comprehensive new agreement on relations between Russia and the EU, and pull out of all preparations for the G8 summit in Sochi in June.