The US chief of naval operations on Monday denounced North Korea's weekend ballistic missile launches and vowed to keep tracking its ships if they are suspected of carrying banned weapons.
"I think they were very unhelpful, and clearly counter to the desires of the international community for a peaceful and stable region," Admiral Gary Roughead told reporters after talks with South Korean military officials.
He said the US Navy would keep tracking North Korean ships suspected of carrying banned weapons.
"As circumstances arise in the future, we will continue to support the resolution and we will conduct operations in support of that."
His comments came as a North Korean ship being tracked by the US Navy headed home after aborting its voyage.
The Kang Nam 1 was the first ship to be shadowed under new UN sanctions imposed on the hardline communist country on June 12 following its nuclear test in May.
South Korean military officials said the Kang Nam 1 appeared to have entered North Korean waters.
"I think what recently happened with the Kang Nam is also a very effective way of stopping that proliferation, and so we saw that ship go back," Roughead said.
The Kang Nam 1, which left North Korea on June 17, was originally reported to be bound for Myanmar.
Pyongyang has responded defiantly to the latest sanctions, vowing to build more nuclear bombs and to hit back against any attempt to search its vessels. On Saturday it test-fired seven missiles in an apparent show of strength.
The tests, staged on the US Independence Day holiday, were seen as a gesture of defiance to Washington as it seeks tough enforcement of the sanctions aimed at shutting down the North's nuclear and missile programmes.
The Security Council will soon discuss how to respond to the weekend missile launches which violated UN resolutions, Seoul's chief nuclear negotiator Wi Sung-Lac said Monday after talks with his Japanese counterpart Akitaka Saiki.
"The UN Security Council will discuss the matter. The level of its response remains to be seen," Yonhap news agency quoted Wi as saying.
The North quit six-nation nuclear disarmament talks in April in protest at the Council's decision to censure its rocket launch earlier that month.
The North has spent an estimated 700 million dollars this year on nuclear and missile tests, enough to solve its food shortage for at least two years, South Korean news reports said on Monday.
The figure includes the estimated 43-million-dollar cost of test-firing five Scud and two Rodong missiles Saturday, according to unidentified government officials quoted by Chosun Ilbo newspaper.
Officials quoted by Chosun estimated it cost 300 million dollars to launch a long-range Taepodong-2 missile on April 5, and another 10 million to launch 10 short-range missiles in recent weeks.
In addition, they estimated the May 25 underground nuclear test -- the country's second since 2006 -- cost between 300-400 million dollars.
JoongAng Ilbo gave similar figures. Neither paper gave the methodology for calculating the costs.
Chosun quoted an unidentified official as saying the North could have bought one million tons of rice on the international market for 300 million dollars, enough to solve its food problem for one year.
The United Nations World Food Programme has said that according to a study last year, nearly nine million North Koreans -- more than a third of the country's 24 million people -- are estimated to need food aid.
US Vice President Joseph Biden on Sunday dismissed the launches as "like almost attention-seeking behaviour" and said the focus was on further isolating Pyongyang.
"We have succeeded in uniting the most important and critical countries to North Korea on a common path of further isolating North Korea," he told ABC television, referring to Russia and China.
US and South Korean officials believe ailing leader Kim Jong-Il, 67, is staging a show of strength to bolster his authority as he tries to put in place a succession plan involving his youngest son Jong-Un.