MOSCOW —Russia has urged Ukrainian authorities to negotiate a cease-fire, after separatist rebels in southeast Ukraine retreated to the two largest cities they control, Donetsk and Luhansk. But Ukraine has vowed to continue its offensive, saying rebels must first lay down their arms.

Separatist rebels are reinforcing blockades and checkpoints as a government military advance forced them to flee their stronghold of Slovyansk.

The surprise retreat came after Ukraine’s President Petro Poroshenko ended a 10-day cease-fire and ordered troops to retake rebel-held territory. Some rebel groups had refused to honor the cease-fire.

The European Union is threatening further sanctions against Russia if it does not do more to rein in the rebels.

But Russian Foreign Minister Sergei Lavrov dismissed concerns about expanded EU sanctions.

He said in regards to Ukraine, he has stopped following EU decisions on black, gray or differently colored lists or deadlines the bloc imposes. He said they do not interest Russia. Above all else, he stressed, Russia is interested in the need to stop the bloodshed.

Ukraine’s Foreign Minister Pavlo Klimkin did not dismiss the possibility of a negotiated cease-fire. He cited discussions between representatives of Ukraine, Russia and the rebels mediated by the Organization for Security and Cooperation in Europe.

Klimkin said they need a consistent and clear signal, including from Russia, that the separatists will talk to reach a bilateral cease-fire. The trilateral contact group, he said, is trying to do everything possible to continue this rather difficult dialogue to save people’s lives.

But Ukrainian authorities want the separatists to first lay down their weapons, many of which Kyiv says are supplied by Russia.

Guessing game

Kyiv is also bolstered by the separatist retreat after months of deadlock with the rebels.

But some analysts say Slovyansk was strategically important only if Russia planned a full-on invasion of Ukraine.

Stanislav Belkovsky, founder and director of the Moscow-based Institute of National Strategy, said such a scenario now seems less likely.

“But after [President] Vladimir Putin finally decided, in my opinion, not to invade Ukraine officially, and act only through separatists and through partisan troops, supporting them in different forms, so … Slovyansk has lost its significance.  And, so all the forces are concentrated to Donetsk and Luhansk. And, President Poroshenko has already proclaimed that those cities would not be bombed,” said Belkovsky.

Political analysts agree a cease-fire is in both Ukraine and Russia’s interest, though Moscow wants a prolonged one that gives it more leverage.

But many agree if a cease-fire is not achieved soon, there is a higher risk Russia may get involved more directly with its military to prevent the rebellion from collapsing.

Defense analyst Pavel Felgengauer is a columnist with Moscow’s Novaya Gazeta newspaper. He said Putin could decide to act before being hampered by bad weather and troop rotations.

“So right now, beginning from the 13th, 14th of July till most likely mid-September, [is] the most dangerous time when Russia has the capability to go in and only needs a political decision to do so,” said the analyst.

Felgengauer added that Russia could choose to conduct air force overflights of Ukraine’s Donbas region (reference to Ukraine’s industrial east) to warn Kyiv off further action against the rebels.

“Because without air support, it is clear that even the introduction of heavy weapons cannot keep the military balance right now. So, that is the last step before actually going in full force with a peace-keeping operation in Donbas, it is the introduction of [the] Russian air force. I think that is right now [in] the cards, though maybe not immediately, likely next week,” said Felgengauer.

But such aggressive military maneuvers would raise calls for further sanctions against Russia, which are already pushing its economy into a likely recession.

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