An apparent incursion of tanks and person­-
nel carriers into southeastern Ukraine on Monday, one day before a high-stakes summit, demonstrated just how difficult it will be for Ukraine to reestablish control over its own territory.

Even as Russian-backed separatists are losing ground on
the battlefield, provocations launched from the Russian side of the two countries’ long border remain a constant threat.

Monday’s column of military vehicles — which Moscow denied sending — appeared near the Sea of Azov, well to the south of the rebel-held city of Donetsk. It came just days after Russia sent a convoy of humanitarian aid trucks — whose purpose, even now, is murky — into Ukrainian territory, provoking international condemnation. The Russian government said Monday that it would send a second convoy despite the chaos the first had wrought.

The bold moves in recent days have complicated the already tense situation in Ukraine, which has been rent by a civil war that has left 2,000 dead in the past five months.

And it raises doubts that Russian President Vladi­mir Putin and Ukrainian President Petro Poroshenko will be able to make any headway toward a peace agreement Tuesday, when they head to Minsk, Belarus, to attend a summit with other European leaders to discuss the conflict.

Russian officials had raised the possibility that Putin and Poroshenko could hold a private bilateral meeting on the sidelines of the summit, and both sides have expressed hopes for peace, to varying degrees.

But if the two leaders do speak privately, lingering suspicions and accusations surrounding Russia’s recent actions at and across the Ukrainian border are likely to complicate the greater discussion about Ukraine’s future.

The summit was set to talk about free-trade issues between the two countries in light of Ukraine’s pending affiliation with the European Union, but likely will be consumed by the war. At the bargaining table, Russia wants recognition of its annexation of Crimea, rights for Russian-speaking minorities and a cease-fire; Ukraine wants Crimea returned, tighter borders and an end to the uprising.

Keith Darden, a Ukraine expert and associate professor at American University, said that Russia’s military incursions have been a demonstration of force as Moscow pushes for a federalized Ukraine — with autonomy granted to the regions. That demand is anathema to Poroshenko’s government. Moscow also insists that Ukraine make no agreement with the North Atlantic Treaty Organization.

“I think with the escalation we’ve seen in the last 24 hours, they’re not intending to wind it down; they’re continuing a strategy in which they’re trying to enfeeble the Ukrainian government until it collapses on its own. Diplomacy isn’t going to pull a rabbit out of a hat this week,” Darden said.

Poroshenko — a billionaire who has been in office only a few months — complicated matters on the eve of the summit by announcing via Twitter that he was dissolving the parliament in advance of elections scheduled for later this fall in an attempt to get a more like-minded legislature. The president noted in a statement that it would be impossible to win a war with a legislative body that could barely agree on whether the separatists were terrorists.

The timing of his announcement was not expected, but parliament had prepared for new elections when two parties agreed in July to withdraw their support for the government.

Russian tanks enter Ukraine

The question of Ukraine’s relations with Europe sparked the protests that began in November after then-President Viktor Yanukovych backed off a plan to sign an association agreement with the European Union. In February, as clashes between protesters and police grew deadly, Yanukovych fled to Russia.

Almost immediately, Russia moved to seize Crimea, and in the spring a pro-Russian separatist movement gained steam in the east.

Since then, Ukraine has regularly accused Russia of supporting and even driving the separatist uprising, with cross-border incursions and shelling. The conflict, coupled with the downing of a Malaysia Airlines flight in July over rebel-held territory, resulted in a chill between Russia and the West not seen since the Cold War.

Steven Pifer, who was U.S. ambassador to Ukraine during the Clinton administration, said that the success or failure of the summit will depend a great deal on “what kind of attitude Putin walks into the meeting with.” Clever diplomats ought to be able to assemble a package of concessions on trade and natural resources that would enable Putin to “extract himself from this mess . . . if he wants a way out,” Pifer said.

Putin’s motives have grown more opaque as tensions have escalated sharply in recent days, following Russia’s dispatching into Ukraine the convoy of trucks — ostensibly bearing supplies for civilians in war-torn areas — without the Kiev government's permission. On Monday, Russian Foreign Minister Sergei Lavrov announced plans for a second convoy, even though the initial foray was characterized by Ukraine as a “direct invasion.”

On Monday, Ukrainian military spokesman Andriy Lysenko said Russian military vehicles flying separatist flags “violated the state border of Ukraine” near Novoazovsk in the southern part of the Donetsk region and engaged in firefights with Ukrainian forces. If true, it would be one of the first times that Russia had penetrated the Ukrainian border outside friendly rebel territory. Lavrov said he had not heard of the incursion and generally dismissed such accusations as “misinformation.”

One witness in the area, who asked not to be identified, said his village was awakened early Monday by the rumble of tanks coming from the east. He said residents ran to meet the soldiers, who were wearing camouflage, speaking Russian and politely asked for water and to borrow their local cellphones.

Yet even those fighting were wondering about the motives behind the sudden appearance of the tanks. Semyon Semenchenko, the commander of a pro-government volunteer battalion in the Donetsk region, called the situation a “small local invasion” and suggested in a television interview that the Russians may be trying to “put the pressure on our president” in advance of Tuesday’s meetings.

Demirjian reported from Moscow. Alex Ryabchyn in Kiev contributed to this report.