SEOUL/MANILA (REUTERS) - US Secretary of State Rex Tillerson held a door open for dialogue with North Korea on Monday (Aug 7), saying Washington was willing to talk to Pyongyang if it ended a series of recent missile test launches.

Tillerson's comments, made at a regional security forum in Manila, were the latest US attempt to rein in Pyongyang's nuclear and missile program after months of tough talk from US President Donald Trump.

The United Nations Security Council on Saturday (Aug 5) imposed its toughest round of sanctions yet against Pyongyang over its two intercontinental ballistic missile tests in July.

But Tillerson appeared conciliatory on Monday.

"When the conditions are right, then we can sit and have a dialogue around the future of North Korea so they feel secure and prosper economically," Tillerson told reporters.

"The best signal that North Korea can give us that they are prepared to talk would be to stop these missile launches," he said, adding that "other means of communications" were open to Pyongyang. "We've not had an extended period of time where they have not taken some type of provocative action by launching ballistic missiles."

Tillerson said Washington would not "specify a specific number of days or weeks" before deciding that North Korea had indeed halted its tests.

The Trump administration's attempts to pressure North Korea into abandoning its nuclear ambitions have so far gained little traction, and Pyongyang has only stepped up its tests, launching two intercontinental ballistic missile tests last month.

Tillerson's remarks might be an attempt to try another tack by the United States, which also has tried to get Pyongyang's ally China to use its influence to prevent North Korean leader Kim Jong Un from building a nuclear arsenal.

The United States has remained technically at war with North Korea since the 1950-53 Korean conflict ended in an armistice rather than a peace treaty. The past six decades have been punctuated by periodic rises in antagonism and rhetoric that have always stopped short of a resumption of active hostilities.

Tensions rose sharply after North Korea conducted two nuclear weapons tests last year and a series of ballistic missile tests.

The UN Security Council unanimously imposed new sanctions on North Korea on Saturday aimed at pressuring Pyongyang to end its nuclear programme. The sanctions could further choke North Korea's struggling economy by slashing its US$3 billion annual export revenue by a third.

North Korea responded robustly and in traditional fashion on Monday, saying the U.N. moves were unwarranted and unfair, and it was ready to teach the United States a "severe lesson" if it attacked.

Tillerson said the support of China and Russia for the latest sanctions sent a strong message to North Korea about what was expected of it.

South Korean President Moon Jae-in and Trump agreed to apply maximum pressure and sanctions on North Korea in a telephone call on Monday, while China expressed hope that North and South Korea could resume contact soon.

The UN resolution bans North Korean exports of coal, iron, iron ore, lead, lead ore and seafood. It also prohibits countries from increasing the current numbers of North Korean labourers working abroad, bans new joint ventures with North Korea and any new investment in current joint ventures.

North Korea said the sanctions infringed on its sovereignty, and vowed to take "righteous action", according to the North's official news agency.

In a statement to the Manila forum on Monday, Pyongyang said it would never place its nuclear programme on the negotiating table as long as the United States maintained a hostile policy against the North.

In a transcript of a statement by Foreign Minister Ri Yong-ho, which was distributed to media in the Philippine capital, Pyongyang called the new UN sanctions "fabricated"and warned there would be "strong follow-up measures". It noted its intercontinental ballistic missile tests last month proved the entire United States was in its firing range, and those missiles were a legitimate means of self-defence.

North Korea has long accused the United States and South Korea of escalating tensions by conducting military drills.

Chinese Foreign Minister Wang Yi said North Korea's statement on the UN sanctions was expected given the North's position. "The key point is that we cannot allow the situation to escalate and we must find a way to reverse the situation amid the current crisis," he said.

He said it is only fair that North Korea sit with other foreign ministers at the Association of Southeast Asian Nations (ASEAN) to discuss UN Security Council sanctions. "We can voice our own opinions, as can North Korea," he said.

Wang also said the core of the Korean Peninsula issue is a security issue, not an economic issue, as North Korea believes it faces an external security threat, while other parties believe Pyongyang's nuclear and missiles programmes are a threat.

During an hour-long phone call, Moon and Trump said they would continue cooperating to rein in North Korea, particularly ahead of a regular joint military drill set for late August, South Korean presidential office spokesman Park Su-hyun told a media briefing.

Moon also was cited as saying there was a need to show North Korea dialogue is still possible should Pyongyang give up its nuclear programme.

Separately, the White House said the two leaders "affirmed that North Korea poses a grave and growing direct threat" to most countries around the world. In a Twitter post, Trump said he was "very happy and impressed with 15-0 United Nations vote"on the sanctions.