"The United States and its allies in NATO are carrying out unplanned exercises in the Black Sea. Not only is a rather powerful naval group involved in these exercises, but also aviation, including strategic aviation. This is a serious challenge for us," he said in an interview with state broadcaster Vesti.

His comments came after Washington this week raised alarms over Russian military activities close to Ukraine's border.

Putin said that his defence ministry "had a proposal to hold its own unplanned exercises in the same waters."

"But I believe that this is inappropriate and there is no need to further escalate the situation there," he said.

On Wednesday, US top diplomat Antony Blinken warned Moscow against making another "serious mistake" on Ukraine as Washington sought clarity about troop movements near the border.

The EU on Friday said it was also concerned by Russian military activities close to Ukraine's border. The 27-nation bloc said it was monitoring the situation with partners including the US and Britain.

The accusations come amid rising tensions between the EU and Moscow over a migrant stand-off on the border between Poland and Kremlin-allied Belarus.

Putin brushed off the accusations, saying "he had not seen these alarmist statements yet."

But he criticised Ukraine for using Turkish-made drones against pro-Russia rebels, saying the move was "strictly" against a peace protocol signed in Minsk in 2015.

Last month, Kiev released footage of what it said was its first use of the Turkish-made TB2 Bayraktar drone against separatists in the east of the country.

"But no one even reacts to this, and the US practically supported it," Putin said.

France and Germany -- both mediators of the conflict -- had criticised the move. Ukraine said it was "defending its territory."

"In parallel to this, they organised drills on the Black Sea. It seems that we are simply not allowed to let our guard down," the longtime Russian leader said.

"Well, they should know that we are not letting our guard down," he added.

The Kremlin had warned that the use of Turkish-made drones -- a potential game-changer in the years-long war -- could escalate the conflict.

Putin said Saturday that Russia is often accused of not fulfilling the Minsk agreements.

"When we ask our partners," he said, "what exactly Russia should be doing according to the Minsk agreement, there is no answer."

In 2014, Moscow annexed Kiev's Crimea peninsula and since then Ukraine has fought a conflict with pro-Russia rebels in the east of the country. The conflict has claimed more than 13,000 lives.