In unusual criticism of a presidential candidate, Pavel, chairman of the NATO Military Committee, said in an interview that Russian "President (Vladimir) Putin and some others may be pleased by this approach" and "to take such an approach would be a great mistake."
Trump, the presumptive Republican nominee in the Nov. 8 US presidential election, has criticized the decades-old NATO alliance with mainly European nations - a cornerstone of US foreign policy - as obsolete and too costly for the United States.
The North Atlantic Treaty Organization was set up in a different era, Trump has said, when the main threat to the West was the Soviet Union. It was ill-suited to fighting terrorism.
Pavel, a former Czech Republic army chief, said the NATO alliance formed in 1949 was not perfect, but it had great potential as well as the chance to be improved, according to Reuters.
"Statements like these are not necessarily damaging, but they are not useful," Pavel said in Singapore on the sidelines of the Shangri-La Dialogue, Asia's biggest security summit.
On Thursday, Democratic presidential frontrunner Hillary Clinton also lambasted Trump's foreign policy platform as "dangerously incoherent".
Trump's emergence as a strong presidential candidate has been a talking point at the Shangri-La Dialogue. Carl Thayer, an Australian security expert, said the prospect of Trump in office would have to be dealt with realistically.