KIEV — Government troops ousted pro-Russian separatists and raised the national flag over a port city in the east and regained control of a stretch of the border with Russia, Ukrainian officials announced Friday.
The pre-dawn attack on separatist strongholds in the city of Mariupol, the second-largest city in the Donetsk region, was over in less than six hours, interior minister Arsen Avakov said. At least five separatists were reported killed, and four Ukrainian soldiers were wounded.
Mariupol is considered strategically important because it is situated on major roads and because steel is exported through its port. Separatists have infiltrated Mariupol several times in the conflict, and full Ukrainian control might prove to be only temporary. But in a sign that Ukrainians expect to stay in charge, President Petro Poroshenko ordered Serhiy Taruta, the Donetsk governor who has been ruling from Kiev in recent weeks, to relocate immediately to Mariupol.
Meanwhile, the State Department confirmed Friday that Russia has sent tanks and other heavy weapons to separatists in Ukraine.
A convoy of three T-64 tanks, several BM-21 “Grad” multiple rocket launchers, and other military vehicles crossed the border near the Ukrainian town of Snizhne, State Department officials said. Reports and images of the weapons’ presence circulated Thursday, but there were conflicting claims about where they had come from.
“This is unacceptable,” said Marie Harf, deputy State Department spokeswoman. “A failure by Russia to de-escalate this situation will lead to additional costs.”
A Western official said intelligence about the movement of the tanks and other weapons into Ukraine was shared Friday with NATO allies. Secretary of State John Kerry complained this week about the flow of Russian arms to separatists in Ukraine in a phone call to Sergey V. Lavrov, the Russian foreign minister.
The T-64 is an obsolescent tank no longer in active use by Russian forces, but it is kept in storage in southwest Russia.
“Russia will claim these tanks were taken from Ukrainian forces, but no Ukrainian tank units have been operating in that area,” the State Department said Friday afternoon. “We are confident that these tanks came from Russia.”
“We also have information that Russia has accumulated multiple rocket launchers at this same deployment site in southwest Russia, and these rocket launchers also recently departed,” the State Department added. “Internet video has shown what we believe to be these same rocket launchers traveling through Luhansk.”
After a series of setbacks, the battlefield victory in Mariupol underscores a growing confidence among Ukrainian officials that the tide may have turned in the conflict that has raged since April.
Poroshenko has called on the separatists to lay down their arms and said he will negotiate with anyone who doesn’t have the blood of innocents on his hands. At the same time, the army has been increasingly aggressive in attacking separatists, who control a swath of the Donetsk and Luhansk regions, where they have proclaimed ‘‘People’s Republics’’ and have asked for international recognition from several countries such as Russia and Venezuela.
Avakov also said Friday that government troops again controlled a 75-mile stretch of the 1,250-mile border that had been in the hands of insurgents.
An aide, Anton Gerashchenko, predicted that the entire border, ‘‘which remained naked after massive terrorist attacks,’’ would be under Ukrainian control by this weekend.
‘‘If they attempt to infiltrate, the militants will be destroyed,’’ he said.
The border, which was considered porous even before the conflict began, has been easily crossed by fighters and convoys bearing supplies.
Some Ukrainian officials are looking for ways to fortify border security once the conflict is under control. Igor Kolomoisky, a billionaire who was appointed governor of the region around Dnipropetrovsk, has urged Poroshenko to begin building an impermeable fence along the border in the regions of Donetsk, Luhansk, and Kharkiv.
He compared it to the Mannerheim Line, which Finland erected along its border with the Soviet Union in the 1920s as a bulwark against the Communists who came to power in the Bolshevik Revolution. He also noted that Israel has built a wall separating it from Palestinians in the West Bank.
Kolomoisky apparently has given the idea of a Ukrainian wall some thought. He proposed an electrified fence be constructed of high-strength steel topped by barbed wire with moats on either side to deter civilians and animals. He estimated that the fence would cost the cash-strapped state at least $70 million, or perhaps double that.
But first, Ukraine must regain control of the breakaway regions.
Government accounts of Friday’s battle to retake Mariupol, if accurate, would suggest that not all of the insurgents are ferociously committed to their cause.
Gerashchenko told reporters that about 30 separatist fighters were detained. Many were hiding in buildings and basements, he said, and were warned that if they didn’t surrender, troops would hurl in hand grenades.