The UN Security Council has condemned "in the strongest terms" a deadly attack in Donetsk, a rebel-controlled city in eastern Ukraine.
Thirteen people were killed and some 20 wounded in the shelling of a bus stop on January 22.
Kyiv and the pro-Russian separatists blamed each other for the attack.
The Security Council called for an independent probe and for those found responsible to be brought to justice.
In Washington, a U.S. State Department spokeswoman said the incident was being investigated by the Organization for Security and Cooperation in Europe.
Jen Psaki also said it showed the need to implement an agreement on establishing security zones between pro-Russian fighters and Kyiv's forces reached in talks involving Russia, Ukraine, France and Germany in Berlin on January 21.
Psaki said Ukraine had the right to defend its own territory and that Russia and Russian-backed separatists were responsible for the vast majority of violations.
Ukrainian President Petro Poroshenko said on January 22 that pro-Russian rebels have paid a "high price" for their escalating attacks on government troops across the separatist-controlled east.
Poroshenko made the statement at an emergency security meeting convened after Kyiv's decision to give up its months-long defense of Donetsk airport.
Earlier, pro-Russian rebels paraded captured Ukrainian soldiers at the bus stop hit in the attack.
The carnage came a day after 16 wounded Ukrainian soldiers were taken prisoner amid the government’s withdrawal from key positions at the Donetsk airport.
Reports say residents hurled glass and shouted abuse at the captive Ukrainian soldiers.
Moscow and Kyiv traded accusations over the bus stop blast, which witnesses said was caused by a mortar or artillery shell.
Prime Minister Arseniy Yatsenyuk, speaking at a Kyiv ceremony marking Ukraine’s National Unity Day, blamed pro-Russian separatists and said Russia should bear responsibility.
Russian Foreign Minister Sergei Lavrov described the attack as a "monstrous new crime" by Ukrainian forces and a "crude provocation" aimed at undermining peace efforts.
Kyiv says the 16 injured Ukrainian soldiers were captured late on January 21 when government forces withdrew from the ruins of the Donetsk airport’s new terminal and from positions near the old terminal building.
Both terminals have been the scene of fierce fighting for months.
Violence continued at the airport on January 22, with Kyiv saying its troops still held parts of the facility.
Ukrainian military spokesman Vladyslav Seleznyov said the decision was made to withdraw 20 Ukrainian soldiers because “their positions had been destroyed and exposed to direct fire,” leading to the deaths of six soldiers during the previous 24 hours.
He said the new terminal building “looks like a sieve and there’s simply nowhere to hide there.”
In the last week, about 100 wounded government soldiers were evacuated from the airport’s new terminal building.
The blast at the bus stop came hours after a meeting in Berlin that German Foreign Minister Frank-Walter Steinmeier said had produced an agreement for the rebels and government forces to pull back heavy weaponry 15 kilometers from a demarcation line established under a cease-fire deal agreed in Minsk in September.