(Refiles to fix typo in "port nearby" in first paragraph)
SANTIAGO, March 16 (Reuters) - Striking workers at BHP Billiton's Escondida copper mine in Chile, the world's largest, are blocking attempts by the company to renew operations at a key port nearby, BHP and an umbrella union said on Thursday, as the stoppage enters its sixth week.
The company said on Tuesday it was gradually resuming operations at Escondida after the 2,500-member Escondida No. 1 Union, which has been on strike since Feb. 9, turned down three offers to return to the negotiating table.
The company would at first restart operations unrelated to the negotiations, it said, before beginning some maintenance operations, and finally resuming copper production, which has been halted since workers walked off.
But the striking workers blocked access to Coloso, a BHP-controlled port near the city of Antofagasta used to export copper, when replacement workers tried to enter it on Wednesday. The blockade continued on Thursday.
"We roundly reject the various actions that the Escondida mine is taking to break the unity demonstrated by the members of the union," Gustavo Tapia, president of Chile's FMC mining umbrella union, said in a statement.
The president of Escondida, a BHP representative, told a local newspaper that the company would insist on accessing the port, and later the mine itself, which is 170 miles southeast in Chile's high desert.
He said if the company could not restart all of its operations, a partial resumption was possible.
Under Chilean law, BHP was allowed to hire temporary workers 15 days after the strike started but said it would wait for 30 days to show its commitment to dialogue. Thursday marked day 36 of the strike.
The strike at Escondida, as well as stoppages at Freeport-McMoran Inc's Grasberg mine in Indonesia and its Cerro Verde mine in Peru have pushed up global copper prices amid supply concerns.
The union repeatedly has said it will not return to the table until BHP agrees not to trim benefits in the existing contract, not to make shift patterns more taxing, and to offer the same benefits to new workers as those already at the mine.
Escondida produced slightly more than 1 million tonnes of copper in 2016, making it the world's largest copper mine.
Rio Tinto and Japanese companies such as Mitsubishi Corp hold minority interests in the deposit. (Reporting by Fabian Cambero and Gram Slattery; Editing by Bill Trott)