North Korea's Vice Foreign Minister Han Song-Ryol told the BBC that missiles would continue to be tested on "a weekly, monthly and yearly basis".
He said that an "all-out war" would result if the United States took military action.
Xi, however, has yet to meet North Korea's young leader Kim Jong Un since the Chinese president took power in 2012 and ties between the two countries have become more distant since Pyongyang began accelerating its missile and nuclear programs since Kim became leader at the end of 2011.
The Trump administration is determined to bring an end to North Korean weapons testing, but Pyongyang refuses to back down.
In an interview with the LA Times, the former secretary said if there is going to be a military reaction from the North, it would be "not a nuclear attack as they have threatened, rather a conventional but still quite destructive attack against South Korea".
How did we get here?
A shocking North Korean video shows a nuclear bomb appearing to strike an American city, before a huge fiery mushroom cloud soars into the sky.
Mattis did not identify the type of missile but said it was not of intercontinental range, meaning it could not reach US territory.
UN Security Council adopts statement condemning NK missile launch
Most North Korean missiles use liquid propellant, which generally must be added to the missile on the launch pad before firing. The latest North Korea missile test failed nine minutes into flight, making it only 40 miles before pinwheeling into the sea.
Pence said in Seoul earlier on Tuesday the Trump administration would review and reform a 5-year-old free trade agreement with South Korea.
Shortly after, North Korea issued a forceful response to the deployment of a USA naval strike group, including the 97,000-ton carrier, the USS Carl Vinson, to the region April 10, saying it would counter "reckless acts of aggression" with "whatever methods the U.S. wants to take". Both missiles failed, exploding shortly after launch at Sinpo.
"But the fact is that they have the overwhelming leverage over the North Korea regime", Prime Minister Malcolm Turnbull told reporters Tuesday in Canberra, Australia's capital.
Speaking alongside South Korea's acting President Hwang Kyo-ahn on Monday, Mr Pence said North Korea should not test US President Donald Trump.
A U.S. State Department official said Monday there is no hiccup in the ongoing deployment of a cutting-edge missile defense system in South Korea, dismissing concern about a political setback. "The United States will strengthen its presence in the Asia Pacific", he said. The U.S. had been considering military options against North Korea because of its nuclear program.
North Korea will continue to test missiles despite the growing tensions between the USA and its allies and Pyongyang, a senior North Korea official said Monday.
The two agreed to develop the deal "in a mutually beneficial way", according to South Korea. "If the United States attacked North Korea in response to a test, the U.S. would be the aggressor".
But her statement in parliament reported by Jiji Press and public broadcaster NHK is likely to be controversial in South Korea.
Lavrov says Russia, US agree strikes on Syria should not be repeated
Tillerson's mission sees the Trump administration taking on the traditional USA role as spokesman for a unified Western position. The U.S. accuses the Syrian government of deliberately launching the deadly chemical attack in Khan Sheikhoun on April 4.
"It will instead seek a bilateral deal with Japan which it hopes will give U.S. companies better access to its market".
And what about other world powers?
"Any kind of action on North Korea will have repercussions on China", our correspondent said.
Meanwhile Chinese foreign minister Wang Yi made a new appeal for calm on the Korean Peninsula and says he believed the United States would prefer a diplomatic resolution to the stand-off.
"China's position on this issue is clear and unshakable, similarly to the position of other countries".
Japanese Prime Minister Shinzo Abe on Monday told a parliamentary session that diplomatic efforts were "important to maintain peace", but "dialogue for the sake of having dialogue is meaningless".
He added that Japan needed to apply pressure on Pyongyang to "seriously respond to a dialogue" with the global community.
Mike Pence fires a fresh warning shot at North Korea
He sounded the warning on Wednesday from the deck of the aircraft carrier USS Ronald Reagan , docked in Yokosuka, Japan. At a briefing later in the day, Seoul played down fears about a possible renegotiation of the bilateral trade deal.