Ukraine “appears to have the momentum” in its surprise invasion of a border region in Russia – but the situation there is “unstable”, a journalist has said.

Speaking to Sky News from Ukraine’s Sumy region, Dan Sabbagh, defence and security editor at The Guardian, said Russian military bloggers are reporting that Ukrainian troops have been able to “make further incursions” of several kilometres in different areas.

“Ukraine still very much appears to have the momentum in this surprise incursion,” he said.

Mr Sabbagh said “several thousand troops” are being committed to the effort by Kyiv.

“But the situation is inherently unstable,” he added.

“War is unpredictable at the best of times, but we really don’t know how hard and how coordinated the Russian response will be.”

Ukrainians want to ‘stick it to Russia’

Mr Sabbagh added that the invasion of Russian territory was being viewed “very positively” inside Ukraine.

He said residents in Sumy, which borders Russia, are “very enthusiastic” and want Moscow to be “put under pressure”.

“They feel very strongly, look, it’s the Russians that invaded us, so why can’t we… stick it to them, if you like.”

Vladimir Putin hasn’t come up with a “hammer blow” of a response to the invasion so far – and the longer it goes on, the “more damaging” it is for his credibility, Mr Sabbagh said.

“Putin’s position remains pretty strong inside Russia. This will certainly chip away at that and he’ll be wanting to do something about that.”