"There are no grounds for panic," Poroshenko said at a meeting in Kiev with heads of security and law enforcement forces, according to the statement posted on the official presidential website. "In two recent months, we have elaborated and implemented serious steps on the preparation of our armed forces and other units to defend Ukraine."
Four Ukrainian servicemen were killed and several wounded Thursday in clashes with the separatists, officials said.
He would not say how many casualties his group had suffered defending the last site held by government troops in the city. But he insisted the separatists' attacks had intensified in recent days.
Several thousand people have been killed in the months-long fighting in eastern Ukraine, which has driven hundreds of thousands of people from their homes to seek safety elsewhere in Ukraine and in Russia.
On Wednesday, NATO's top commander accused Russia of sending armored columns and an unspecified number of troops into eastern Ukraine in the preceding two days.
The Russian Foreign Ministry denied the report.
NATO had previously accused the Russians of moving troops and equipment across the border.
"We observe them to rotate their forces from time to time," said Butusov, editor of Tsenzor.net, a popular online publication. "They are keeping up their military presence in the region to assist the separatists to continue hold their ground."