European Union leaders have resisted Ukraine's demands for peacekeepers, as monitors reported a surge in shelling near a strategic government-held city in the east.
At an EU-Ukraine summit in Kyiv, Brussels did, however, agree to boost humanitarian support as Kyiv fights pro-Moscow separatists.
Observers from the Organization for Security and Cooperation in Europe (OSCE) say shelling continues in the east despite a cease-fire deal signed in the Belarusian capital Minsk in February.
Ukraine's pro-Western President Petro Poroshenko called on the EU officials "to deploy an international peacekeeping mission in our country which will contribute to the complete fulfilment of the Minsk accords."
But Donald Tusk, the president of the European Council of EU leaders, said after the summit, "We know about Ukrainian expectations today, but it's impossible to send a military mission."
Tusk did say, however, that the EU would "send as soon as possible a civilian assessment mission... to assess the humanitarian situation" in Ukraine.
The conflict between government troops and pro-Russian rebels has killed more than 6,100 people in the past year and displaced more than 1 million, according to the United Nations.
The EU leaders pressured Ukraine to speed up reforms to eradicate corruption in Ukrainian politics and business in return for closer ties with the bloc.
Tusk, a former Polish prime minister, said in Ukrainian on arrival in Kyiv, "You have a powerful enemy, but you also have a lot of friends. You can count on their help, but it will not be enough unless you yourself change Ukraine."
Poroshenko, whose government highlighted recent reforms such as the setting up of a new anticorruption bureau, said he was aiming for Ukraine to be ready to apply for EU membership within five years.
But he warned the country first needs more help to stabilize the east, where pro-Russian rebels have seized partial control of two regions and are exchanging shell fire with government forces.
Western officials say Russia is sending arms and troops to the separatists, a charge Moscow has repeatedly denied.
The OSCE said its monitors on April 26 witnessed "the most intense shelling" near the flashpoint town of Shyrokyne since fighting began there in mid-February, as well as the movement of heavy weapons.
The OSCE said in a statement that they "observed sporadic to continuous exchanges of fire involving small arms, machine guns, rocket propelled grenade and automatic grenade launchers" throughout April 26.
Shyrokyne lies a few kilometers from the port of Mariupol, the biggest city still under Kyiv's control in the conflict zone.
Separatists have threatened a new offensive against Mariupol after commemorations of the Soviet Union's victory in World War II on May 9 are out of the way.
In Washington, U.S. State Department spokesman Jeff Rathke said that Russia has deployed more air defense systems into eastern Ukraine and positioned several near the front lines.
Prime Minister Arseniy Yatsenyuk said during the talks that Ukraine wants to see "European partners and friends among shareholders" of Ukraine's gas transportation system.
Some 40 percent of Russian gas imports to Europe pass through Ukrainian territory.
Russia threatened last autumn to suspend gas supplies to Ukraine citing Kyiv's multibillion-dollar debt.
The EU stepped in to prevent a cut-off of supplies to Europe.
