The United States has seen no signs of North Korea planning a nuclear test, Washington's chief nuclear negotiator said Tuesday.

Amb. Sung Kim, special representative for North Korea policy, made the remark after South Korea's main intelligence agency reported to lawmakers earlier in the day that the North is preparing for its fourth nuclear test, although a test is not imminent.

"I have seen nothing to indicate that a test is being planned.

We are monitoring closely, as I said," Kim told reporters after a Senate Foreign Relations Committee hearing.

"North Korea should not be conducting any nuclear test for in fact they should not be conducting any nuclear activity. Multiple UN Security Council resolutions prohibit them from carrying out nuclear activities. I certainly hope that they are not planning to conduct another nuclear test," he said.

North Korea has conducted three underground nuclear tests so far, in 2006, 2009 and 2013. The country has also conducted a series of long-range missile or rocket launches since 1998. In its most recent launch in late 2012, the North put a satellite into orbit.

Analysts have warned that it is only a matter of time until the North develops nuclear-tipped missiles. Some experts have recently warned that the communist nation's nuclear arsenal could expand to as many as 100 warheads by 2020. (Yonhap)