BRUSSELS (Reuters) - Ukraine's counter-offensive against Russian forces is proving difficult due to landmines and other obstacles but Ukrainian forces are right to proceed cautiously, NATO's top military official said on Monday.

"The counter offensive, it is difficult," said Admiral Rob Bauer, a Dutch military officer who is the chair of NATO's military committee.

"People should never think that this is an easy walkover. It will never be," he told reporters at NATO headquarters in Brussels.

Ukrainian forces face defensive obstacles sometimes up to 30 kilometres deep as they attempt to break through Russian lines, Bauer said, drawing on a historic comparison to make his point.

"We saw in Normandy in the Second World War that it took seven, eight, nine weeks for the allies to actually break through the defensive lines of the Germans. And so, it is not a surprise that it is not going fast," he added.

Satellite images reviewed by Reuters in April showed Russia had built extensive fortifications, trenches, anti-vehicle barriers and other obstacles to slow any Ukrainian advance.

Bauer's comments echoed remarks by General Mark Milley, the top U.S. military officer, who said on Friday the counter-offensive would be very difficult, very long and "very, very bloody".

Bauer said Ukrainian forces were right to be cautious to avoid high casualties as they probed for possible breakthroughs.

"It is extremely difficult, this type of operation and I think the way they do it is commendable," he said.

He said Ukrainian forces should not face pressure or criticism for not moving more quickly.

"This is a very, very difficult time for them," Bauer said.

(Reporting by Andrew Gray; Editing by Frank Jack Daniel)