Troops from Russia and Uzbekistan began joint military drills on Monday near the Afghan border amid fears in both countries over a worsening security situation in Afghanistan that could spill over into Central Asia, Reuters reported.

Russia said 1,500 Russian and Uzbek troops would take part in the five-day exercises that began at the Termez military site in Uzbekistan, the TASS news agency reported.

In a sign of how serious Moscow is taking the potential threat from Afghanistan, it said it would send a much bigger military contingent to Tajikistan for separate trilateral exercises due to begin there later this week, Reuters reported.

Those separate drills are due to take place on Aug. 5-10 and involve Russian, Uzbek and Tajik forces.

Security has rapidly deteriorated in Afghanistan amid a U.S. troop withdrawal. Moscow fears that could destabilise its southern defensive flank and push refugees into its Central Asian backyard.

On Monday, Russia’s defence ministry said that 1,800 of its soldiers would take part in the Tajik drills, instead of 1,000 as initially planned. More than 2,500 troops would be involved in total, it said, Reuters reported.

Moscow will also use 420 units of military hardware for the drills, double the quantity originally planned, it said.