China told its citizens on Thursday they were not safe in the Philippines and its state media warned of war, as a month-long row over rival claims in the South China Sea threatened to spill out of control.

Chinese travel agencies announced they had suspended tours to the Philippines, under government orders, and the embassy in Manila advised its nationals already in the country to stay indoors ahead of planned protests.

"Avoid going out at all if possible, and if not, to avoid going out alone. If you come across any demonstrations, leave the area, do not stay to watch," the embassy's advisory said.

The safety alerts came as government-controlled media in China warned the country was prepared to go to war to end the stand-off over Scarborough Shoal -- small islands in the South China Sea that both nations claim as their own.

"No matter how willing we are to discuss the issue, the current Philippine leadership is intent on pressing us into a corner where there is no other option left but the use of arms," the China Daily said in an editorial.

"Since ancient times, our nation has deemed war the last resort in handling state-to-state relations. But Manila is living in a fantasy world if it mistakes our forbearance for timidity."

The editorial echoed other warnings in recent weeks in the state-run media that China was prepared to use its massive military advantage to crush the Philippines' challenge for the shoal.

The two nations have had non-military vessels stationed at the shoal since April 8 in an effort to assert their sovereignty to the area.

The dispute began when Philippine authorities detected Chinese ships fishing there. They attempted to arrest the crew, but were blocked by Chinese surveillance vessels that were quickly deployed to the area.

The shoal sits about 230 kilometres (140 miles) from the Philippines' main island of Luzon. The nearest major Chinese landmass is 1,200 kilometres northwest of the shoal, according to Filipino navy maps.

The Philippines insists its claims to the area are backed by international law, as the shoal is well within its 200-nautical-mile exclusive economic zone.

But China claims virtually all of the South China Sea as its territory, even waters close to the coasts of the Philippines and other Asian countries.

Taiwan, Brunei, Vietnam and Malaysia also claim parts of the sea, which is believed to sit atop vast oil and gas resources. The rival claims have for decades made the waters one of Asia's potential military flashpoints.

More than 70 Vietnamese sailors were killed in 1988 when China and Vietnam battled for control of the Spratlys, an archipelago south of Scarborough Shoal.

The Philippines, which admits to having an extremely weak and poorly resourced military, has repeatedly said it wants to solve the stand-off over the shoal through diplomatic means.

But it has also said it secured a pledge from the United States, its main military ally, to protect the Philippines from attacks in the South China Sea.

A coalition of Filipino activist groups is planning to hold rallies at Chinese embassies around the world on Friday to support the Philippines in the dispute.

Organisers are hoping thousands of people will attend what they expect to be the biggest of the rallies, in Manila, and the Chinese embassy's safety alert was circulated chiefly to warn its nationals about that protest.

But Jackson Gan, a Filipino-Chinese businessman who is one the rally's organisers, said there was no need for such a warning because the protest would not target individuals and there had been no inciting of violence.

"This is going to be peaceful. No burning of Chinese flags, just singing of patriotic songs and making our presence felt," Gan told AFP.