Indonesia will mobilize fishermen to join warships in the South China Sea and help defend against Chinese vessels, the government in Jakarta said on Monday, as the biggest standoff for years between Southeast Asia's largest country and China escalated off the coast.

In an unusually strong statement, President Joko Widodo told reporters: "There is no negotiation when it comes to our sovereignty."

The standoff that has continued since last month in the northern Natuna islands, where a Chinese coast guard vessel is accompanying Chinese fishing vessels, has soured the generally friendly relationship between Jakarta and Beijing.