French President Emmanuel Macron expressed his "extreme concern" about Russian strikes on Ukraine on Monday, during a call with his Ukrainian counterpart Volodymyr Zelensky and promised that France would increase military aid.

"The president spoke of his extreme concern about strikes that have caused civilian victims," Macron's office said in a statement.

"He reaffirmed his full and complete support for President Zelensky and the commitment of France to increase its support for Ukraine, in line with the needs expressed by Kyiv, including in terms of military equipment."

France has faced repeated criticism over its lower level of military support to Ukraine compared with allies.

According to an August ranking by the Kiel Institute for the World Economy, France's 233 million euros ($230 million) of military aid place it 11th in the world, well behind the United States (25 billion euros), Britain (four billion euros) and Poland (1.8 billion euros).

But officials and experts say capacity rather than political will is at the root of the differences, and French contributions to European Union funds for Ukraine should be included in the overall figure for its aid.

Paris has sent 18 of its highly prized truck-mounted Caesar cannons to Ukraine and is planning to deliver more of the artillery pieces.