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.

Coordinated suicide bomb blasts at Kabul airport on Thursday have left at least 170 dead and many more injured.

Isis-K has claimed responsibility for the attack, which targeted US troops and Afghans attempting to secure the last remaining places on military evacuation flights out of the country.

On Friday, the US said it killed a "planner" for the group in an air strike in Afghanistan.

The threat of further attacks around Kabul Airport will increase as Western troops get closer to leaving the country, Defence Secretary Ben Wallace said on Friday, with Isis-K hoping to show it drove foreign troops from Afghanistan.

"The threat is obviously going to grow the closer we get to leaving," he told Sky News. "The narrative is always going to be, as we leave, certain groups such as ISIS will want to stake a claim that they have driven out the US or the UK." 

On Thursday night, President Joe Biden pledged to "hunt down" the attackers and ordered his military to plan strikes.

He said: "Know this: we will not forgive, we will not forget. We will hunt you down and make you pay." 

Here we look at how Isis-K came to exist and why it poses such a threat to US forces in Afghanistan. 

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.