KYIV: Separatists in Ukraine asked Moscow to help repel "aggression" on Wednesday and explosions rocked the breakaway eastern city of Donetsk as the United States warned everything is in place for a major attack by Russia on its neighbor.

Hours after the breakaway regions issued their plea, at least five explosions were heard in the separatist-held eastern Ukraine city of Donetsk on Thursday morning, a Reuters witness said. Four military trucks were seen heading to the scene.

Airlines should stop flying over any part of Ukraine because of the risk of an unintended shootdown or a cyber attack targeting air traffic control amid tensions with Russia, a conflict zone monitor said.

"Russia has positioned its forces at the final point of readiness," U.S. Secretary of State Antony Blinken told NBC Nightly News. "Everything seems to be in place for Russia to engage in a major aggression against Ukraine."

The United Nations Security Council will discuss the stand-off on Wednesday night, diplomats said, in the 15-member body's second late night meeting on the crisis this week.

Treasury Secretary Janet Yellen spoke separately with her counterparts from Britain and Canada, with officials planning to stay in close touch on potential further responses. G7 leaders are also due to talk on Thursday.

Convoys of military equipment including nine tanks were seen moving towards Donetsk earlier on Wednesday from the direction of the Russian border, a Reuters witness reported.

Shelling has intensified since Russian President Vladimir Putin recognised two separatist regions as independent and ordered the deployment of what he called peacekeepers, a move the West calls the start of an invasion.

Moscow has long denied that it has plans to invade despite massing tens of thousands of troops near its neighbor.

"I am asking for help to repel the Ukrainian regime's military aggression against the population of the Donetsk People's Republic," said Denis Pushilin who heads the area Moscow recognised as independent, according to TASS news agency.

The White House rejected the separatist plea for help as another Russian "false flag" operation, a fake crisis manufactured to justify greater intervention.

"This is an example of it," White House press secretary Jen Psaki told reporters. "That is suggesting that they feel under threat. By whom? The Ukrainians that the Russians are threatening to attack?"

U.N. chief Antonio Guterres has dismissed claims of a genocide in eastern Ukraine. Under international law, genocide is an intent to destroy, in whole or in part, a national, ethnical, racial or religious group.

WAR FOOTING?

A total of 80% of the soldiers assembled are in a position to launch a full-scale invasion on Ukraine, a senior U.S. defence official said.

Satellite imagery taken on Wednesday showed new deployments in western Russia, many of them within 10 miles (16 km) of the border with Ukraine and less than 50 miles from the Ukrainian city of Kharkiv, U.S. satellite company Maxar said.

The images showed field deployment, military convoys, artillery and armoured personnel carriers with support equipment and troops. The images could not be independently verified by Reuters.

Russia has partially closed airspace in the Rostov flight information region to the east of its border with Ukraine "in order to provide safety" for civil aviation flights, according to its notice to airmen.

A 30-day state of emergency in Ukraine restricting the freedom of movement of conscripted reservists, curbing the media and imposing personal document checks, according to a draft text, begins on Thursday.

The Ukrainian government has also announced compulsory military service for all men of fighting age.

While the West has held off the most stringent sanctions measures it could impose, the United States stepped up the pressure by imposing sanctions on the firm building the Nord Stream 2 gas pipeline and its corporate officers.

Germany on Tuesday froze approvals for the pipeline, which has been built but was not yet in operation, amid concerns it could allow Moscow to weaponize energy supplies to Europe.

The rouble plunged more than 3% as the European Union blacklisted Russian lawmakers, freezing their assets and banning travel. Wall Street slipped on news of security measures taken by Ukraine's government. Read full storyRead full story

'SILENCE' ON TALKS

Ukrainian government websites, which have experienced outages in recent weeks blamed by Kyiv on cyber attacks, were again offline on Wednesday. Ukraine's parliament, cabinet and foreign ministry websites were affected.

Moscow has described warnings of an invasion as anti-Russian hysteria. On Wednesday, it took down flags from its embassy in Kyiv, having ordered its diplomats to evacuate for safety reasons.

Ukraine's military said one soldier had been killed and six wounded in increased shelling by pro-Russian separatists using heavy artillery, mortar bombs and Grad rocket systems in the two breakaway areas in the last 24 hours.

Russia says the crisis is a dispute with the West, demanding security guarantees, including a promise never to allow Ukraine to join NATO.

But the recognition of the separatist regions was accompanied by much stronger language against Ukraine, which Putin called an artificial construct wrongly carved out of Russia by its enemies.

While both sides have said they are still open to diplomacy to resolve the crisis, Ukrainian President Volodymyr Zelenskiy said Moscow had approved an offensive and not replied to an invitation for talks.

"Today I initiated a telephone conversation with the President of the Russian Federation. The result was silence," he said.