On Monday, US President Donald Trump said the status quo with nuclear-armed North Korea was unacceptable, calling for new sanctions on the country.
An unnamed North Korean Foreign Ministry spokesman said the USA administration's policy to maximize pressure on North Korea was "little short of lighting the fuse of total war", the state news agency reported Tuesday.
Flurry of diplomacy under way as North Korea reportedly marks military anniversary with large-scale artillery drill.
Xi urged "restraint" on North Korea in the call, according to the Chinese foreign ministry.
The submarine's arrival comes as the USS Carl Vinson aircraft carrier heads towards the Korean Peninsula in a show of force.
In a statement from the South Korean Joint Chiefs of Staff, South Korea said it was monitoring the situation and remained "firmly prepared".
U.S. Senator Lindsey Graham said he and fellow Republican John McCain had dinner with Trump on Monday and discussed North Korea. "The North Korean nuclear and missile problem is an extremely serious security threat to not only the worldwide community but also our country", the Japanese leader told reporters in Tokyo afterward.
Commentators said that Trump was backing himself into a corner over North Korea.
A guided-missile submarine, it can carry up to 154 Tomahawk cruise missiles and is able to stealthily deploy up to 66 Navy SEALs or other special operations troops.
There's also a possibility that North Korea, facing potential changes in regional dynamics as Washington presses Beijing to pressure Pyongyang more aggressively, opts to mark the anniversary with a missile launch of lesser magnitude.
Angered by the approach of the carrier group, which could arrive within days, North Korea said the deployment of the USS Carl Vinson was "an extremely unsafe act by those who plan a nuclear war to invade".
It is also believed to be preparing a sixth nuclear weapons test and recently detained a USA citizen, the third held in the country. The U.S. 7th Fleet said two American destroyers were conducting simultaneous maritime exercises with naval ships from South Korea and Japan.
The firing exercise took place on the anniversary of the founding of the North Korean army, amid heightened tension on the Korean peninsula over the North's development of weapons. The reasons for the detention of Tony Kim, who taught accounting at the Pyongyang University of Science and Technology, weren't immediately clear.
But the timing, as the U.S. increases its military presence in the area, allows North Korea to remind its opponents that it could cause crippling damage with conventional artillery to highly populated areas in South Korea.
The parade also featured previously unseen large rocket canisters and transporter erector launcher trucks, or TELs.
In March, North Korean state media reported that the country successfully conducted a ground test of a new high-thrust rocket engine, which it said was a breakthrough for its space program and efforts to create "Korean-style strategic weapons". This indicated that North Korea is developing technologies to "cold-launch" ICBMs, ejecting them from the launch tubes before they ignite in midair, which would prevent its limited number of ICBM-capable launcher trucks from being damaged and also allow the missiles to be fired from silos.
"We need to move more quickly and with greater determination to convince North Korea either to pursue denuclearisation or to apply enough pressure that it stops those activities", Toner told a telephone news briefing.
Experts say these missiles could one day be capable of hitting targets as far as the continental United States, although North Korea has yet to flight test them.