MEXICO CITY, March 29 (Reuters) - Mexican state oil giant Pemex said on Tuesday that engineers were in the process of restarting a catalytic plant at the company's Salamanca oil refinery.

The plant at Pemex's fourth biggest refinery went offline at 5.15 a.m. ET on Tuesday morning, Pemex said in a statement.

It was not yet clear whether output had been affected, a Pemex spokesman said.

The Salamanca refinery, in central Mexico, is capable of producing up to 220,000 barrels of oil per day.

(Reporting by Adriana Barrera; Editing by Savio D'Souza)