Ukraine said on Friday it had destroyed part of a Russian military convoy that crossed onto its territory in an incursion that has sent cross-border tensions rocketing.

NATO accused Russia of active involvement in the "destabilization" of eastern Ukraine, where pro-Kremlin separatists have been fighting against Kiev for four months.

Britain has summoned Moscow's ambassador to London over reports of a Russian military incursion across its border with Ukraine, the Foreign Office said Friday.

Alexander Yakovenko was summoned "to clarify reports of Russian military incursion into Ukraine and continued buildup of equipment on border," the ministry said in a tweet.

NATO chief Anders Fogh Rasmussen confirmed Friday an incursion by Russian armored vehicles into Ukraine, saying it showed Moscow's continued efforts to "destabilize" its western neighbor.

"I can confirm that last night we saw a Russian incursion, crossing of the Ukrainian border," he told journalists in Copenhagen after meeting with the Danish defense minister.

Ukrainian officials were preparing to inspect a massive Russian "aid" convoy bound for the conflict-torn east on Friday after Russian armored vehicles crossed the border, fueling fears Moscow is trying to bolster the unraveling insurgency.

The Ukrainian military had announced that checks had begun on the near 300-truck convoy but later said only that 59 border and customs officials had arrived at a Russian border post to prepare to carry out the inspections.

An alleged Russian intelligence colonel with an obsession for reenacting historical battles, Igor Strelkov, who quit as a key rebel chief on Thursday, was a driving force behind the brutal pro-Moscow insurgency tearing apart east Ukraine.

The Russian citizen -- whose real name is Igor Girkin -- is a fierce nationalist and Orthodox Christian believer who reportedly honed his skills fighting in conflicts from Bosnia to Moldova's breakaway territory of Transdniestr and Chechnya, before helping Moscow seize Ukraine's Crimea peninsula in March.

Pro-Russian rebels in eastern Ukraine suffered dramatic setbacks Thursday as top military chiefs quit and Ukraine's forces pummeled their strongholds, cutting off a major center from the Russian border.

Kiev's advance came as a massive Russian "humanitarian" convoy parked up close to the frontier, with doubts still swirling over whether the trucks would be allowed to cross.

Ukraine's parliament on Thursday approved a law paving the way for a string of sanctions on Russian businesses and nationals accused of backing a separatist uprising in the east of the country.

Kiev has drawn up a sanctions list of 65 mainly Russian companies and 172 individuals and the new law now gives Ukraine's National Security and Defense Council -- headed by the president -- the right to impose the punitive measures.

President Vladimir Putin said on Thursday that Russia should not "fence itself off from the outside world" despite a plunge in East-West relations over the pro-Kremlin insurgency in Ukraine.

Putin added during a visit to Crimea -- seized by Russia from Ukraine in March -- that an ongoing trade war with the United States and Europe did not mean Moscow "should break ties with partners. But we should also not let them treat us with disdain."

Hackers broke into Russian Prime Minister Dmitry Medvedev's Twitter account on Thursday, tweeting his resignation and criticizing President Vladimir Putin.

"I'm resigning. I'm ashamed of the government's actions. Forgive me," said Medvedev's Russian-language Twitter account.

NATO chief Anders Fogh Rasmussen expressed concern on Wednesday that Russian President Vladimir Putin's ambition went "beyond Ukraine," where he is accused of stoking a bloody rebellion by pro-Kremlin separatists.