Ukrainian troops along the de facto border with Crimea and facing rebels in the east have been put on high alert.

It follows accusations by Moscow on Wednesday that Kiev was carrying out armed incursions into Crimea.

Ukraine's envoy to the UN asked Russia to provide evidence of the claims, and said some 40,000 Russian troops were massed on the Crimea-Ukraine border.

His Russian counterpart told the UN Security Council of Moscow's "concern and outrage" at the alleged incursions.

Crimea was annexed by Russia in 2014 after an unrecognised referendum.

The two sides are accusing each other of destabilising the situation as Ukraine prepares to celebrate the 25th anniversary of independence on 24 August, and Russia prepares for a parliamentary election on 18 September.

'Bad intentions'

Meanwhile, Russian state TV has broadcast an apparent confession by a man named as Yevhen Panov, saying he was part of a Ukrainian defence ministry force sent into Crimea "to carry out acts of sabotage". He says the group also included officers of Ukraine's military intelligence.

Reports in Ukraine say Mr Panov was a former volunteer fighter who has more recently been associated with a charitable organisation.

In other developments:

The US state department has urged both sides to tone down their rhetoric saying it has "the ability to escalate what is already a very tense situation"

Crimean leader Sergei Aksyonov has accused the US of being behind the alleged armed incursions saying "Ukrainian officials wouldn't have had the courage for such actions"

Russia has announced its two newest ships in the Black Sea Fleet - armed with cruise missiles - will begin exercises in the eastern part of the Mediterranean from 15 August

At a meeting of the UN Security Council in New York, Ukraine ambassador Volodymyr Yelchenko said he had urged his Russian counterpart to produce proof of Ukrainian incursions into Crimea.

"If it happened in reality, where are the proofs? Statements, pictures, photos, videos, whatever," he told reporters. "They are only words".

He also said the massing of so many troops by Russia in Crimea "might reflect some very bad intentions".

But Russia's UN ambassador Vitaly Churkin said that "instead of counting our military they should be bringing an end to the conflict" in eastern Ukraine.

Both envoys told the UN they hoped that tensions would not escalate further.

'Dangerous game'

Russia's FSB intelligence agency on Wednesday said it had smashed a Ukrainian military intelligence network attempting incursions into Crimea over the weekend. "Heavy fire" from Ukrainian territory resulted in the death of a Russian soldier. An FSB employee died while trying to detain infiltrators, it said.

President Putin accused Kiev of "practising terror" and vowed to "not let such things slide by", saying: "This is a very dangerous game".

President Poroshenko described the accusations as "preposterous and cynical", and said: "These fantasies are only a pretext for making more military threats against Ukraine".

More than 9,500 people are reported to have been killed in the conflict in eastern Ukraine that followed the Russian annexation of Crimea.

The annexation, which was itself largely bloodless, followed a vote to secede at a referendum on self-determination on 16 March 2014 - a ballot which was not recognised internationally.

The US and European Union imposed sanctions on Moscow in response to its seizure of the territory.