UNITED NATIONS (AP) _ A U.N. official said Thursday the problem of mines, bombs, missiles and grenades strewn across Kuwait was ''extremely grave'' and that Iraqi floating mines might drift across the Persian Gulf to other nations.

Undersecretary-General Martti Ahtisaari said Kuwaitis could be killed and maimed for years by unexploded ordnance. He asked the United Nations and individual nations to begin an urgent program to help clear the explosives.

Ahtisaari told the Security Council that Iraq deliberate tried to erase Kuwait's national identity following the invasion and occupation of the emirate on Aug. 2, but that Kuwait has begun a strong recovery in just a month.

In reporting on his visit to Kuwait this week, his assessment of Kuwait's condition was far more optimistic than his report last week on Iraq. He said ''near apocalyptic'' allied air raids had reduced Iraq to a ''pre- industrial'' economy.

The undersecretary-general noted that in Kuwait, however, ''both urban and outlying areas are reported to have a plethora of live bombs, rockets, grenades and other unexploded devices. The authorities also stated that explosive booby traps had been scattered.''

He said ''many thousands'' of non-metallic mines were sowed on and around beaches, and on the Kuwaiti border with Saudi Arabia. Plastic mines are more difficult to detect.

These mines are reported to be able to float, so ''the future location of at least some may be anticipated to depend on tide ands currents, and to present a threat to the coasts of other gulf states,'' he found.

Some stretches of the Saudi-Kuwaiti border are so heavily mined that they may need to be permanently fenced off.

Ahtisaari said that without an immediate clearance program, ''acute danger could remain for years to come. In my opinion, this may be one of the most urgent humanitarian needs, as summer begins and the population, adults and children, begins to return to their country from exile.''

At least two-thirds of Kuwait's pre-invasion population of about 1.7 million fled the country, he said.

Ahtisaari said he found the Kuwaitis ''have fresh memory of a brutal occupation and the sacking and pillaging of their homes, their resources and their environment.''

''I and my mission saw for ourselves prolific evidence of arson, looting, malicious destruction of homes, businesses, markets, museums, libraries, and all that a nation holds dear,'' he added.

The official noted that pollution from the hundreds of burning oil wells set afire by the Iraqis poses ''still-uncharted perils to health.'' he said.

''There can be no doubt that a deliberate attempt was made to extinguish Kuwait, its national identity, the pride of its people in their history and achievements,'' he said.

''The manner of destruction, with its coordinated vandalism and massive looting, leaves an indelible image.''

But he found that ''the process of repair and replacement has already proceeded to such a degree that ... (the Kuwaiti people) are fully capable of providing adequate support for humanitarian needs.''