KINGSTON, Jamaica (AP) _ Grenada, still occupied by some 600 U.S. and Caribbean military personnel, will join the three-year-old eastern Caribbean security alliance, Prime Minister Herbert A. Blaize said Tuesday.

He made the announcement at meeting with leaders of the six other island nations in the alliance. Their talks followed a summit conference attended by Canadian Prime Minister Brian Mulroney and chiefs of state of 11 English- speaking Caribbean nations.

The security alliance has dealt mainly with cooperation on such matters as coast guard rescue missions, but the members have said they want it to be developed as a combat force to oppose foreign-inspired insurrection or incursions by mercenaries.

The prime minister of Barbados, Tom Adams, said the leaders also discussed the security situation in Grenada. About 300 soldiers or policemen of the Caribbean security alliance and some 300 U.S. military personnel are still in Grenada as Blaize rebuilds the police force following the U.S. invasion in October 1983.

The invasion came a week after Prime Minister Maurice Bishop was slain on Oct. 19 in a bloody feud within his pro-Cuban, leftist government.

A 16-man junta of Marxist army officers seized power and then was ousted by the U.S. invaders and small contingents of military and police units from some members of the eastern Caribbean security alliance.

Blaize led his centrist party to victory in Grenada's elections last December.

Adams said the alliance leaders agreed on a timetable for the withdrawal of foreign forces from Grenada, ''but don't want to publicize it for security reasons.''

He said the U.S. and Caribbean withdrawals would ''not necessarily'' be at the same time. U.S. officials have indicated all U.S. military units would leave the island within nine months.

Blaize said Grenada's ''security and police needs are being reviewed right now.

''Our police are being trained and equipped. They are more advanced this month. Next month, they will be even more advanced.''

But he declined to say exactly when the police force of about 500 men would be prepared to take over security duties. The 2,000-man army was disbanded following the invasion.

Adams had indicated before the talks that the islands might seek more assistance from the United States and Canada to improve their security, but he said the leaders decided not to mention that issue in their post-meeting report.

He also said the proposal he made last year for a centrally headquartered regional standing army was ''in a state of flux'' and wasn't discussed Tuesday.

The six nations that formed the eastern Caribbean security alliance are Antigua and Barbuda, Barbados, Dominica, St. Kitts-Nevis, St. Lucia, and St. Vincent and the Grenadines.

Jamaica, which participated in the 1983 invasion and still has some police units in Grenada, has not joined the alliance.