DONETSK, Ukraine -- As separatists conceded that militants from Russia's province of Chechnya had joined the rebellion, a Ukrainian government official cautioned Wednesday that its borders had become a "front line" in the crisis.
Chechnya's Moscow-backed strongman brushed away allegations that he had dispatched paramilitary forces under his command to Ukraine, saying he was powerless to stop fellow Chechens from joining the fight.
While there is no immediate indication that the Kremlin is enabling or supporting combatants from Russia crossing into Ukraine, Moscow may have to dispel suspicions that it is waging a proxy war if it is to avoid more Western sanctions.
In a wide-ranging foreign policy speech at the U.S. Military Academy at West Point, N.Y., President Barack Obama addressed the Ukraine crisis by saying, "Russia's recent actions recall the days when Soviet tanks rolled into Eastern Europe."
The Kremlin welcomed the election Sunday of billionaire Petro Poroshenko as Ukraine's new president. An advocate of strong ties with Europe, Mr. Poroshenko also favors mending relations with Russia. He replaced the pro-Moscow leader who was driven from office in February. That ouster led to Russia's annexation of the Crimean Peninsula in southern Ukraine, which triggered the sanctions, and a violent pro-Moscow insurgency in the east.
Reports circulate almost daily of truckloads of gunmen crossing from Russia, and authorities believe that they are a vital reinforcement to the armed rebel force that has repeatedly thwarted government security operations.
Intense fighting in a successful government offensive Monday to dislodge rebels from the Donetsk airport appeared to have died down, with only sporadic violence reported Wednesday.
Ukrainian border service head Mykola Lytvyn said he has deployed all reserves to the eastern and southern frontiers.
"Our border, especially in the Donetsk and Luhansk regions, has become a front line that various terrorists are trying to break through," he said at a Kiev news conference. "Daily fighting with terrorists and groups of criminals near the Ukrainian and Russian border have become our routine reality."
Russians who cross into Ukraine by road must go through passport and customs control on both sides of the border, a procedure usually requiring several hours.
But these controls would be virtually nonexistent for those who drive across fields.
The Kiev government condemns the roiling insurgency as the work of "terrorists" bent on destroying the country. Rebels insist that they are only protecting the interests of the Russian-speaking population of the east.
Russia denies mass border crossings are occurring, although separatist leaders of the self-proclaimed Donetsk People's Republic now freely admit that their ragtag army has many foreigners, including some from the Russian province of Chechnya.
The Donetsk People's Republic militia is a force of uncertain strength, composed of units of varied provenance and abilities. Many of those questioned insist that they are either local or from Crimea.
Donetsk Mayor Oleksandr Lukyanchenko said some fighters admitted for treatment after Monday's clashes were from cities in Chechnya. It is this Chechen contingent that has aroused the most alarm in Ukraine.
Chechen leader Ramzan Kadyrov was a former rebel who fought Russian forces in the first of two devastating separatist wars and switched sides during the second campaign, when his father became the region's pro-Russia leader.
After his father's death in a rebel bombing, Mr. Kadyrov, 37, stabilized the region, relying on generous Kremlin funding and ruthless paramilitary forces blamed for extrajudicial killings, torture and other abuses.
Mr. Kadyrov's forces, known for their warrior spirit and deadly efficiency, helped Russia win a quick victory in a 2008 war with Georgia. He has vowed an unswerving fealty to Russian President Vladimir Putin and praised his policy in Ukraine.
Mr. Kadyrov has derided allegations that he dispatched militias to Ukraine, but undermined his claim with veiled threats.