Ukraine’s government continued efforts to put down a separatist rebellion in eastern reaches of the country on Wednesday, as military forces battled rebel holdouts near the airport here and the newly elected president was quoted as saying he would seek U.S. military aid to help end the crisis.

Two days after a clash between Ukrainian military forces and pro-Russian militants at Sergei Prokofiev International Airport killed about 50 people, president-elect Petro Poroshenko said the nation’s fight to regain control “has finally really begun.”

But the second abduction in three days of international election observers showed the limits of Kiev’s control. Eleven vote monitors from the Organization for Security and Cooperation in Europe (OSCE) were detained by an unidentified armed group after being stopped at a road block in the town of Marinka, about 20 miles west of Donetsk, OSCE spokesman Michael Bociurkiw said. The monitors, including one American, were released Wednesday evening after about seven hours and escorted back to Donetsk, he said. Four other OSCE monitors are still missing after disappearing near Donetsk on Monday night.

As sporadic exchanges of artillery and machine-gun fire continued in the vicinity of the Donetsk airport terminal and local media reported more shelling in rebel-controlled Slovyansk, Denis Pushilin, a leader of the separatist Donetsk People’s Republic, issued a defiant call to continue resistance.

“We are not going to leave, and we are not going to surrender. This is our land and our home,” Pushilin told a pro-separatist rally of a few hundred people in Donetsk’s Lenin Square. “We are getting stronger and stronger.”

As he spoke, surrounded by bodyguards and wearing a bulletproof vest under his blue jacket, a Ukrainian military jet roared in the distance above the city’s international airport, which has been closed since heavy fighting Monday claimed at least 50 lives.

Pushilin said the rebels would ignore Ukrainian ultimatums to lay down their arms. He also said more volunteers — “our brothers” — were coming over the borders of friendly regions and states into Donetsk as reinforcements.

Some Ukrainian officials — and local residents who have been close enough to hear the accents of fighters — have charged that Russia has enabled battle-hardened fighters from South Ossetia and Chechnya to cross the border and join the conflict. Ramzan Kadyrov, a Chechen regional leader who is an ally of Russian President Vladi­mir Putin, on Wednesday denied sending militants into Ukraine.

“Ukrainian sources are spreading reports that some ‘Chechen units’ from Russia have broken into Donetsk. I am officially declaring that this does not correspond to the facts,” Kadyrov said in an Instagram posting. But he also said some Chechens might have gone to Ukraine on “personal business.”

There was a tense calm Wednesday in the center of Donetsk. There, streets were emptied of traffic, with many stores closing and people getting out of town because of fear that the violence would spread. A spokeswoman for the separatist Donetsk People’s Republic said that there had been no deaths in Donetsk as of late Wednesday afternoon and that separatist forces were in control of the city.

Poroshenko, 48, a billionaire who won the May 25 presidential election in the first round, said in an interview with Germany’s Bild newspaper that he intended to call on the United States for military supplies and training.

But he has also signaled his interest in pursuing talks with Putin, despite his view that Moscow has played a role in instigating the violence.

A Ukrainian Jewish family rescued by Israel from Donetsk says they have mixed feelings about fleeing from their home country. (Reuters)

“Russia’s goal was, and is, to keep Ukraine so unstable that we accept everything that the Russians want,” Poroshenko said in the interview. “I have no doubt that Putin could, with his direct influence, end the fighting.”

Russia on Wednesday appeared to pivot from what was initially a cold but vaguely conciliatory response to Poroshenko’s win to accusing Kiev of escalating the violence.

Russian Foreign Minister Sergei Lavrov told U.S. Secretary
of State John F. Kerry during a telephone conversation that Ukraine’s interim government must stop its military operation in southeastern Ukraine, the Reuters news agency reported, quoting the Russian Foreign Ministry.

Lavrov “underscored the need for an immediate halt by Kiev of the punitive operation in the southeastern regions and for the swiftest possible launch of a nationwide dialogue, including direct talks with representatives of the southeast, in the interests of a peaceful solution,” the ministry said.

In a commencement speech Wednesday at the U.S. Military Academy, President Obama used the example of Ukraine to tout his emphasis on multilateral action. For some critics, “working through international institutions like the U.N., or respecting international law, is a sign of weakness,” he said. “I think they’re wrong.”

“In Ukraine, Russia’s recent actions recall the days when Soviet tanks rolled into Eastern Europe,” Obama said. “But this isn’t the Cold War. Our ability to shape world opinion helped isolate Russia right away.” Speaking to cadets and top Army leaders, Obama said the U.S.-led “mobilization of world opinion and international institutions served as a counterweight to Russian propaganda and Russian troops on the border and armed militias in ski masks.”

Hauslohner reported from Moscow. Michael Birnbaum in Kiev, William Branigin in Washington and Daniela Deane in London contributed to this report.