Romania says a Russian drone has breached its airspace - the second NATO country to report such an incursion.

Romanian fighter jets were in the air monitoring a Russian attack in Ukraine on Saturday and were able to track the drone near Ukraine's southern border, the defence ministry said in a statement.

Ukrainian President Volodymyr Zelensky said the incursion could not be a mistake - it was an obvious expansion of the war by Russia. Moscow has not commented on the Romanian claims.

On Wednesday, Poland said it had shot down at least three Russian drones which had entered its airspace.

In its statement, Romania's defence ministry said it detected the Russian drone when two F-16 jets were monitoring the country's border with Ukraine, after Russian air attacks on Ukrainian infrastructure on the Danube.

The drone was detected 20km (12.4 miles) south-west of the village of Chilia Veche, before disappearing from radar. However, it did not fly over populated areas or pose imminent danger, the ministry said.

Poland also responded to concerns over Russian drones on Saturday, with Prime Minister Donald Tusk announcing that preventative operations of Polish and allied aviation have commenced.

The Czech Republic has also sent a special operations helicopter unit to Poland in response to increasing tensions along NATO's eastern flank.

In response to the latest drone incursion, President Zelensky emphasized that the Russian military is acutely aware of their drone operations.

US President Donald Trump has indicated readiness to impose tougher sanctions on Russia, contingent upon NATO nations reducing their reliance on Russian oil.