WASHINGTON (Reuters) - The FBI grounded crop-dusting planes in the continental United States on Sunday, urging vigilance to ''suspicious activity'' involving hazardous chemicals used in aerial treatment of farm fields.

The FBI, which is investigating the Sept. 11 hijack attacks at the World Trade Center and the Pentagon, did not directly link the grounding to these attacks in a message on the National Agricultural Aviation Association web site.

Under the headline ``Ag Aviators prohibited to fly on Sunday 23,'' the FBI message read, ``Members should be vigilant to any suspicious activity relative to the use, training in or acquisition of dangerous chemicals or airborne applications of same, including threats, unusual purchases, suspicious behavior by employees or customers and unusual contacts with the public.''

An update from the Federal Aviation Administration on the same web site -- www.agaviation.org -- said that crop-dusting operations were prohibited ``for reasons of national security: in the 48 contiguous states through 12:05 A.M. on Monday in each time zone.

FAA spokeswoman Diane Spitaliere confirmed that the statement had been issued on Sunday morning.

A U.S. government spokesman said the FBI's decision to keep the crop-dusters from flying stemmed from ``an abundance of caution.''

Time magazine reported on Saturday that investigators had found a crop-dusting manual during a search for those responsible for the attacks on New York and Washington, triggering concern that crop-dusting planes might be used for chemical or biological assaults.

The FBI in Washington said it had not seen the report and had no comment on it.

The magazine said its issue due out on Monday would disclose that a manual for crop-dusting equipment had been found during a search of the suspected hide-outs of some of those believed responsible for the attacks, which left more than 6,800 people dead or missing.