The Ukrainian president made the remarks during his annual parliamentary address, calling for the country's military to stay vigilant over the "permanent threat" from Russia. He said troops needed to be prepared for a possible "full-scale invasion" by the former Soviet state.
"There remains a colossal threat of resumption of large-scale fighting on the part of Russian terrorist groups," Petro Poroshenko said.
The speech marked one of the first times the Ukrainian strongman used the word "invasion" to describe the continuing conflict in eastern Ukraine. The United Nations estimates more than 6,000 people have died since pro-Russian separatists took control of parts of eastern Ukraine last year - following Russia's annexation of Crimea.
Poroshenko said the number of Russian servicemen deployed along the 2,200-kilometer (1,375-mile) border was "one and a half times greater than a year ago." He announced expenditure on the nation's military would be increased to more than 5 percent of GDP, to combat the insurgency. Ukraine is receiving emergency loans from the International Monetary Fund to stave off bankruptcy.
Surge in violence
Wednesday saw a sudden uptick in fighting, with Ukrainian military officials saying at least five servicemen were killed and almost 40 wounded around the town of Marinka, west of the rebel stronghold of Donetsk. Fighting had decreased since the signing of a ceasefire deal in February, although frequent skirmishes had continued to break out between Kyiv troops and pro-Russian rebels.
The West has long accused Moscow of supporting the rebel fighters, and says it has failed to fully commit to the peace deal's terms. Russia denies involvement, instead alleging Ukraine is inflaming tensions to deter the European Union from removing economic sanctions.
The EU has described the violence as the worst since the Minsk deal was signed, worrying it could spark "a new spiral of violence and suffering."
Ahead of an emergency United Nations Security Council meeting over the Ukraine situation, NATO chief Jens Stoltenberg said that Russia's behavior was showing "increased unpredictability, increased insecurity, increased nervousness."
The issue is also expected to come up at a two-day meeting of the leaders of the Group of Seven (G7) industrialized nations in Germany, a conference Russia has been from attending. German Chancellor Angela Merkel has repeatedly called on Russian President Vladimir Putin to end the conflict and rejoin the group.
It follows after Canadian Prime Minister Stephen Harper said Russia should not be allowed back into the G7 as long as Putin remains in charge.
an/msh (AFP, Reuters, AP)