Russia's foreign minister has accused the United States of encouraging Ukraine to challenge Moscow.
Sergei Lavrov, in televised remarks Saturday, said "our American colleagues still prefer to push the Ukrainian leadership toward a confrontational path."
Lavrov said the chances for settling the Ukrainian crisis would have been better if the negotiations only depended on Russia and its European partners.
On Friday, Ukrainian President Petro Poroshenko extended a unilateral cease-fire with pro-Russian separatists for three more days.
Some of the rebels said they would observe a truce as long as the Ukrainian military does.
Prior to his announcement , Ukraine said four of its soldiers were killed and several wounded in an attack by separatist fighters in Kramotorsk in the eastern part of the Donetsk region.
The European Union is demanding the separatists and their supporters in Moscow take steps to de-escalate the crisis in eastern Ukraine -- including holding "substantial" peace talks and letting a European team monitor the cease-fire.
Friday Ukraine signed a new free trade and economic pact with the EU, angering Russia which is threatening to retaliate.
This is the same deal former pro-Russian president Viktor Yanukovych refused to sign last year, leading to his overthrow and the chaos in eastern Ukraine.
Mr. Poroshenko said he is determined to make the agreement work and is committed to making the necessary economic reforms.
A spokesman for Russian President Vladimir Putin says the Kremlin will have to take steps to protect its economy. He gave no details.