Pro-Russian separatists in eastern Ukraine have suffered dramatic setbacks after top military chiefs quit and Kiev’s forces pummeled their strongholds, cutting off a key rebel-held city from the Russian border.

Kiev’s advance came as the Russian “humanitarian” convoy parked up close to the frontier, with doubts still swirling over what the trucks contained and whether they would be allowed to cross.

According to reports in two British newspapers, a smaller group of Russian vehicles, including armored cars, did cross the border into Ukraine late Thursday.

Igor Strelkov The Daily Telegraph “witnessed a column of vehicles including both armoured personal carriers and soft-skinned lorries crossing into Ukraine at an obscure border crossing near the Russian town of Donetsk.”

The Guardian daily said a column of 23 armoured personnel carriers and other vehicles crossed near Donetsk, about 200 kilometres from the Ukrainian town of the same name.

The separatist leadership has showed signs of unraveling following four months of fighting that have left more than 2,000 dead and many residents in the region without power or running water, and with dwindling food supplies.

The rebels said their main military chief, Igor Strelkov, had resigned while the rebel commander in the second-biggest insurgent stronghold of Lugansk, Valery Bolotov, told Russian television he was “temporarily” stepping down because of earlier injuries.

The announcements came after Ukraine’s military said it had completely surrounded Lugansk, cutting all links to the border with Russia, which Kiev believes has been supplying the insurgents with weapons.

Intense shelling on Lugansk and the main insurgent bastion of Donetsk in eastern Ukraine left more than 25 people dead, while Ukrainian forces reported nine troops dead and 18 injured over the past day.

While reiterating its support for Kiev, Washington urged its ally to exercise restraint and keep civilian casualties to a minimum.