President Petro Poroshenko has vowed to seek Crimea's return to Ukrainian rule, and reiterated that his country faces the threat of a major Russian invasion in the east.

Speaking at a news conference on June 5, a day after warning of a "colossal threat" of the resumption of full-scale fighting with Russian-backed rebels in his annual state-of-the-nation address, Poroshenko said that "every day and every moment, we will do everything to return Crimea to Ukraine."

He vowed to continue working with Western nations to maintain sanctions imposed on Moscow over its annexation of the Black Sea peninsula in March 2014, which followed Russian troops deployments and a hastily-staged referendum condemned as illegitimate by about 100 UN member states.

"It is important not to give Russia a chance to break the world's pro-Ukrainian coalition," Poroshenko said in the televised news conference.

He spoke ahead of a G7 summit on June 7-8 at which industrialized nations are certain to discuss Russia's interference in Ukraine, and a European Union meeting later this month that Kyiv hopes will yield a formal decision tokeep EU sanctions in place through the end of the year.

Amid reports of a Russian military buildup near the border with eastern Ukraine, where Russian-backed separatists hold parts of two provinces, Poroshenko said that the threat of a Russian invasion is "unprecedentedly high."

"Such a number of troops has not been near our borders since August last year and it seems to me there must be some very serious reasons for keeping these large groups of troops there," Poroshenko said.

In his annual address to parliament on June 4, Ukraine's president warned his country that there is a "colossal threat" that large-scale fighting could resume in the east, where fighting between government forces and pro-Russian separatists has killed more than 6,400 since April 2014.

The annual address came a day after Ukraine and the separatists, backed by Russia, traded blame over the biggest outbreak of fighting since all three signed a cease-fire and peace deal brokered by the leaders of Germany and France in Minsk, Belarus, in February.

Speaking of the clashes that focused on the town of Maryinka, west of the rebel-held provincial capital of Donetsk, Poroshenko said the area had been "completely cleansed of saboteurs."

Poroshenko said that five Ukrainian soldiers were killed in the fighting, and claimed that some 80 of their opponents were killed and 12 others -- including one Russian citizen -- were captured.

Russia denies sending its troops or weapons into eastern Ukraine despite what Kyiv and NATO say is growing evidence of direct Russian military involvement.

He said that between 500 and 1,000 separatists took part in what he termed a the attack against Ukrainian positions near Maryinka on June 3. Rebels blame Kyiv for the fighting.

The Ukrainian military said on June 3 that a separatist offensive had forced it to use artillery that had been pllled back from the front lines under the cease-fire deal, which is known as Minsk II.

The fighting on June 3 raised concerns that the cease-fire, which had led to a sharp reduction in fighting but was violated frequently, might fall apart entirely and further reduce hopes for an eventual political settlement.

Poroshenko also said that a "UN Support Office" will be soon set up in Ukraine, calling that "the first step towards having UN Security Council peacekeepers in the country's east."

With reporting by UNIAN