The ships are "strengthening warning and surveillance activities in the waters and airspace around the area," Japan's largest daily newspaper said, according to its English language sister publication, The Japan News.
It comes amid reports that tour companies in China have stopped arranging tour groups to North Korea, which had previously been a popular destination for Chinese tourists.
Media outlet thepaper.cn said several agencies had ceased organising package tours, including travel website Lumama and Ctrip - China's biggest tour agency - which stopped group trips to North Korea at the end of 2016.
Ctrip told the Shanghai-based website that it did not know when it would resume trips to North Korea.
However, travel agencies told thepaper.cn that there had not been a notice from authorities forcing them to cancel trips to the reclusive state.
Media reported last week that China's national carrier, Air China, suspended flights from Beijing to Pyongyang because of dwindling passengers.
Tensions have been escalating in recent weeks. A North Korean missile "blew up almost immediately" on its test launch on Sunday, US officials said.
But despite the apparent setback for Pyongyang, experts in Asia believe Trump has little room for manoeuvre over North Korea, given the military deterrent Kim Jong-un's regime has at its disposal.