NEW YORK (CNN) - Negotiations to avert the first phase of a transit strike in New York and hammer out a contract for 33,000 transport workers resumed on Saturday, according to a spokesman for the Transport Workers Union.

A Transport Workers Union spokesman said talks between the union and the Metropolitan Transit Authority were held at the Grand Hyatt Hotel in Manhattan until they were broken off after four hours. However, union officials said they are available to continue bargaining later today.

Only the primary negotiators have been involved in today's talks, with vice presidents and executive leaders "blacked out" or not invited to the current round of discussions, according to another union official, according to Ainsley Stewart, TWU vice president.

Key issues are remaining on the table such as health benefits, pensions and wages.

If negotiators cannot reach an agreement, the first phase of a strike is scheduled to begin after midnight Monday, union officials said.

A strike, which would come during Christmas shopping season, would sock the New York region's economy heavily reliant on buses and subways. The union and the authority have been working to avoid that scenario.

Negotiations for new contracts for bus and subway workers reached an impasse early Friday after all-night bargaining past the midnight strike deadline.

The union rejected a contract offer from the MTA and called for a strike, first against private bus lines, then later against city transit lines if negotiations remain stalled.