NATO said on Wednesday it had observed columns of Russian military equipment entering southeastern Ukraine since the beginning of the week while the Organization for Security and Cooperation in Europe feared an assault on the Ukrainian port city of Mariupol

Speaking in Sofia, Bulgaria, Supreme Allied Commander Europe General Philip Breedlove said Russian air defenses, artillery, tanks and troops had crossed the border into Ukraine. “We do not have a good picture at this time of how many,” he said. “We agree that there are multiple columns that we have seen.”

Ukraine’s military accused Russia last week of sending dozens of artillery guns and tanks into the Donbas border region where two breakaway republics have requested annexation by Russia.

Breedlove made his comments after a report from the Organization for Security and Cooperation in Europe (OSCE) said its monitors had seen a convoy of unmarked trucks, some towing howitzer artillery pieces and multilaunch rocket systems, entering Donetsk, one of the cities held by the separatists, on Tuesday.

OSCE chief Lamberto Zannier told members of the European Parliament the following day he believed an offensive against Mariupol was imminent. Shelling around the city, which is situated on the Sea of Azov, had been extraordinarily severe, he pointed out, which could be part of preparations for an attack.

Russia, as usual, denied its forces were present in Ukraine.

However, it previously occupied and annexed the Crimean Peninsula from its former Soviet republic and deployed tanks and troops to the country when rebels fighting in the areas around the cities of Donetsk and Luhansk appeared on the verge of being defeated in August. Russian support gave the separatists the upper hand and the military situation in southeastern Ukraine had seemed at a stalemate since.

Taking Mariupol would enable the Russians to build a land bridge to the Crimea. “The region is currently accessible to Russia only by air and across the Black Sea and the Sea of Azov,” writes Robert Coalson for Radio Free Europe/Radio Liberty. “Such a land bridge would make it much easier for Moscow to supply Crimea.”

But it would probably also entail a very bloody campaign, according to Edward W. Walker, a comparative political scientist at the University of California, Berkeley. Mariupol’s residents are staunchly opposed to the prospect of a Russian occupation. The separatists in southeastern Ukraine would be defending an even longer line of control. “And it would mean new rounds of Western economic sanctions,” something Russia can ill afford when both the value of its currency and the price of oil are tumbling, pushing its economy into recession.

Ukrainian forces originally withdrew from Mariupol in May when civil unrest broke out in the city. The army moved back in the following month.