For more than a month, Donetsk has braced for invasion from the east, with tens of thousands of Russian troops massed just over the border and seemingly prepared to overrun this breakaway region of Ukraine.

But confirmation from NATO on Friday that most of those troops had pulled back brought no relief to the edgy streets of Donetsk: The Russians are already here. And the invasion may still come — from Ukrainian forces to the west.

Acting Ukrainian Defense Minister Mykhilo Koval said Friday that military operations in Donetsk and the surrounding areas would continue “until these regions begin to live normally, until there is peace.”

That means taking on a separatist movement that gets more support from Russia by the day, as trucks laden with fighters and weapons rumble across a porous border that is only lightly defended by Ukrainian forces.

Despite president-elect Petro Poroshenko’s promise to crush the rebellion in the east and unite his fractured country in “hours,” even senior Ukrainian officials acknowledge that the armed forces are ill-equipped to counter what amounts to a stealth Russian invasion.

"The problem with the army is that it doesn’t have experience conducting a war,” said Konstantin Batozsky, an adviser to the governor of Donetsk, who in recent days has taken refuge in Kiev. “For 23 years, we were an absolutely peaceful country."

That peace has been obliterated in recent months by what has become a proxy fight between Russia and the West for supremacy in Ukraine.

Donetsk, which has borne the brunt of the violence, was relatively calm Friday after several days of fierce clashes. But the stakes continued to rise as the Organization for Security and Cooperation in Europe announced that a team of international observers had been detained, just days after the abduction of another team.

The first team disappeared Monday night, and the self-appointed mayor of the rebel-held city of Slovyansk said Thursday that his forces were holding its four members captive. There was no immediate word on the whereabouts of the five monitors who vanished Friday in the adjacent region of Luhansk.

Luhansk and Donetsk were declared “sovereign” republics by separatists after a disputed May 11 vote on self-rule. In both places, rebel forces have taken over government buildings and fought off efforts by the security forces to restore state control.

Rebel leaders have called on Russia to invade Ukraine to protect people in the east from their own government. For the moment, that option appears unlikely. NATO Secretary General Anders Fogh Rasmussen said Friday that two-thirds of Russian troops that had been massed on the border had now withdrawn. But, he said, enough troops remain to “take action if a political decision is taken. So we continue to call on Russia to stop supporting armed pro-Russian gangs and seal the border so that we don’t see arms and fighters crossing into Ukraine.”

Ukrainian officials estimate that five to seven trucks slip into the country daily with supplies and reinforcements for the rebels.

Russia strenuously denies any role in the infiltration and has said Ukraine’s security problems are of its own making. Yet in recent days, the Russian role has grown more conspicuous.

On Thursday, rebel leaders acknowledged that 33 fighters killed in an airport shootout with the Ukrainian military were Russian nationals. And early Friday, a Ukrainian border post in Luhansk faced an unusual assault from inside Russia, as 300 militants gathered on the Russian side and launched an attack, according to the border agency.

“They were shooting with grenades and small arms,” the agency said, adding that one soldier was injured.

Within the rebel leadership, commanders who are closely associated with Russia are taking on a more prominent role. The entrance to the rebel-occupied regional administration building in Donetsk is emblazoned with a massive, Hollywood-style poster of Igor Girkin, who goes by the pseudonym Igor Strelkov and is believed by Ukrainian authorities to be a Russian intelligence officer.

Girkin is now defense minister of the self-proclaimed Donetsk People’s Republic and is closely associated with the Vostok Battalion, a well-trained rebel unit that includes many Russian and Chechen fighters. The group seized the administrative building on Thursday, causing dismay among some civilian leaders of the rebellion who feared a coup.

On Friday, the Vostok fighters handed the building back to the rebels’ civilian leaders and their somewhat ragtag band of militants.

Before departing, Vostok arrested several fighters accused of looting and initiated a clean-up of the trash, mangled furniture and unsightly tire-and-barbed-wire barricades left after nearly two months of occupancy by the insurgents.

“Vostok did the right thing,” said a rebel fighter named Vladimir who is not a member of the elite unit but who stands guard in the building’s airy 11th-floor suite of offices. “The people who rob supermarkets need to be punished.”

The baby-faced 26-year-old, who gave only his first name, pointed to the courtyard below, where separatists were sweeping up glass, mowing the lawn and hauling off debris. The People's Republic, he said, intended to set a good example and prove it can govern.

“You will see, it will be beautiful here,” he said, fingering the trigger on his assault rifle. “As long as the Ukrainian army doesn’t bomb.”

Birnbaum reported from Kiev. Karen DeYoung in Singapore and Abigail Hauslohner in Moscow contributed to this report.