Hostilities have simmered for years but tensions are now escalating amid fears of a Russian invasion.

Kyiv, Ukraine – According to Washington, Russia has amassed more than 100,000 Russian soldiers on the border with Ukraine and in annexed Crimea in recent weeks.

This has stoked fears in Kyiv and the West that the Kremlin may start a new war with its neighbour and former province that chose to break away from Moscow’s political orbit.

Earlier this month, a top Ukrainian military expert told Al Jazeera that Russia could invade Ukraine as early as January, unleashing a “brief and victorious” war. But Russia denies it is planning an invasion. Moscow says it can move Russian troops wherever it wants and that any of its acts are defensive. Russian officials, including President Vladimir Putin, have for their part warned NATO against expanding eastwards. So, what is at the heart of the conflict that has been going on for more than seven years?

What is now Ukraine, Russia and neighbouring Belarus were born on the banks of the Dnieper River, almost 1,200 years ago in Kievan Rus, a medieval superpower that included a huge chunk of Eastern Europe. But Russians and Ukrainians parted ways linguistically, historically and, most importantly, politically.

Putin has, however, claimed repeatedly that Russians and Ukrainians are “one people”, part of the “Russian civilisation” that also includes neighbouring Belarus. Ukrainians reject his claims. Ukraine went through two revolutions in 2005 and 2014, both times rejecting Russia’s supremacy and seeking a path to join the European Union and NATO. Putin is particularly enraged by the prospect of NATO bases next to his borders and says Ukraine joining the US-led transatlantic alliance would mark the crossing of a red line.

US President Joe Biden has said he thinks his Russian counterpart Vladimir Putin will "move in" on Ukraine but does not want "full-blown war".

He told a news conference Mr Putin would pay a "serious and dear price" for invading, but indicated a minor incursion might be treated differently. The White House later stressed any Russian military move would be met with a swift, severe response from the West.

Russia has some 100,000 troops near the border but denies planning an invasion. President Putin has made a series of demands to the West, insisting Ukraine is never allowed to join Nato and that the defensive alliance abandons military activity in Eastern Europe.

Talks between the West and Russia have so far failed to reach a breakthrough, with some of Moscow's demands rejected as non-starters. US Secretary of State Antony Blinken is due to meet the Russian foreign minister in Geneva on Friday, having warned that Russia could attack Ukraine "on very short notice". However, at his news conference on Wednesday, Mr Biden said: "There are differences in Nato as to what countries are willing to do, depending on what happens. "If there's Russian forces crossing the border… I think that changes everything.

"What you're going to see is that Russia will be held accountable if it invades and it depends on what it does," he said. "It's one thing if it's a minor incursion, and then we end up having to fight about what to do and not do etc."

White House officials rushed to clarify the US position after Mr Biden's comments. "If any Russian military forces move across the Ukrainian border, that's a renewed invasion, and it will be met with a swift, severe, and united response from the United States and our Allies," press secretary Jen Psaki said. Hours earlier, French President Emmanuel Macron told the European Parliament that Europeans had to conduct their own dialogue with Moscow.

The Biden administration is investigating a recent spate of mysterious health incidents reported by American diplomats and other government employees in Vienna, say US officials.

Some of the symptoms are similar to those first reported by US diplomats and spies in Havana, Cuba, in 2016 and 2017, for which no definitive cause has yet been determined.

US officials said more than 20 new cases are being looked at by medical teams at the State Department and elsewhere, including the Pentagon and the CIA.

"In co-ordination with our partners across the US government, we are vigorously investigating reports of possible unexplained health incidents (UHI) among the US Embassy Vienna community," the State Department said.

"Any employees who reported a possible UHI received immediate and appropriate attention and care."

Some believe the unexplained injuries, which include brain damage, are the result of microwave or radio wave weapons, but despite years of study, there is no consensus as to what or who might be behind the incidents or whether they are actual attacks.

Beijing has been accused of sponsoring an attack on Microsoft Exchange email servers earlier this year which "recklessly" exposed thousands of businesses to criminal hackers.

The Chinese government has been accused of "systematic cyber sabotage" in statements by the UK and allies, including the US and Canada.

The allies are announcing that they believe Chinese state-sponsored hackers were responsible for an attack earlier this year which "indiscriminately" compromised an estimated 400,000 servers worldwide, leaving them exposed to criminals.

More than 70 organisations in the UK were compromised by the hack, perpetrated by a group associated with Beijing according to the National Cyber Security Centre. This attribution has been supported by allies.

Foreign Secretary Dominic Raab described the hacking campaign - which is believed to have compromised the on-premise email servers indiscriminately with an intention to subsequently target specific victims - as "a reckless but familiar pattern of behaviour" from the Chinese government.

Four Iranian intelligence officials have been charged with plotting to kidnap a New York-based journalist critical of Iran, US prosecutors say. The indictment did not name the target, but Masih Alinejad, an Iranian-American author and activist, says it was her.

The conspirators, who all live in Iran and remain at large, also allegedly plotted to lure a person in the UK and three others in Canada to Iran. All of the targets had been critical of Iran, according to the indictment.

The US justice department says the Iranian officials sought to lure the New York-based journalist to a third country where the abduction was planned. The plotters even offered money to the writer's relatives in Iran to betray them, which they refused to do, the indictment contends.

They hired private investigators to spy on the target's Brooklyn home and family, and set up a live video feed of the property. They also researched a service offering military-style speedboats for evacuation from New York City, and maritime routes to Venezuela, an ally of Iran.

There was no immediate response from the Iranian government.