A Russian invasion of Ukraine could happen at any moment, a defence minister has warned.

Armed Forces Minister James Heappey has described the massing of roughly 130,000 Russian troops along the Ukrainian border as a "credible force set to do something imminently".  It comes after Defence Secretary Ben Wallace compared diplomatic efforts to prevent a Russian invasion of Ukraine to appeasement as he said it is "highly likely" Vladimir Putin will order an attack despite the concerted talks to avert war.

Military coups have been a regular occurrence in Africa in the decades since independence and there is now concern they are becoming more frequent.

Sudan has this year experienced two such events, one in September which failed and the latest in which Gen Abdel Fattah Burhan dissolved the civilian arm of a transitional government and took over.

In Guinea, President Condé was ousted by the army in September and in neighbouring Mali there have been two interventions by the army in less than a year, the most recent one in May.

In Niger, a coup was thwarted in March just days before a presidential inauguration.

So are military interventions occurring more often on the continent?

The exercises, involving ground troops and air forces, come amid growing instability in Afghanistan.

Chinese and Russian military forces are engaged in joint exercises in north-western China as ties grow between the two autocratic states amid uncertainty over instability in Afghanistan.

The exercises involving ground troops and air forces are due to continue through until Friday in the Ningxia Hui Autonomous region.

The region borders on Xinjiang, where China has detained more than one million Uyghurs and members of other Muslim minorities in what it calls a campaign against terrorism and extremism.

Xinjiang shares a narrow frontier with Afghanistan, and Beijing is concerned about violence spilling over its border if the Taliban take control in the country following the pullout of US and NATO troops.

While not part of a formal alliance, Russia and China have aligned their military and foreign policies largely in opposition to those of the US and its allies.

The official Xinhua News Agency said the exercises began on Monday and were presided over by Li Zuocheng, a member of the ruling Communist Party's Central Military Commission.

The exercise aims to "deepen the joint anti-terrorism operations between the Chinese and Russian militaries and demonstrate the firm determination and strength of the two countries to jointly safeguard international and regional security and stability", Xinhua said, citing Chinese and Russian officials.

"It reflects the new height of the China-Russia comprehensive strategic partnership of co-ordination for a new era and of the strategic mutual trust, pragmatic exchanges and co-ordination between the two countries," Xinhua said.

Russia has backed China in its claim to virtually the entire South China Sea, over which Beijing clashed with common rival the US at a high-level UN Security Council meeting on maritime security.