SINGAPORE - South-east Asian countries would face long-term problems that could last for decades if Muslim militants are able to escalate their ongoing siege on the southern Philippine city of Marawi.

That was the consensus of defence ministers who met over lunch on the sidelines of the Shangri-La Dialogue, Defence Minister Ng Eng Hen told reporters after hosting the luncheon for 22 visiting ministers and their representatives on Saturday (June 3).

About 400 militants linked to the Islamic State in Iraq and Syria (ISIS) terror group overran Marawi on May 23, and Philippine forces have deployed troops and airpower but are still trying to retake the city.

Dr Ng said that if the situation "was allowed to even escalate or entrench it would pose decades of problems for Asean cities and interests of other countries within Asean".

"All of us recognise that if not addressed adequately, it can prove a pulling ground for would-be jihadists who can launch attacks from there," he added.

The Marawi crisis also underscored the importance of initiatives such as the Sulu Sea patrols, which will be jointly conducted by Indonesia, Malaysia and the Philippines in the waters off southern Philippines.

He added that many countries, including Singapore, have pledged support to help the Philippines deal with the problem.

"If they need help, we stand ready to help, whether it's information, intelligence, or otherwise," he said.

Malaysian Defence Minister Hishammuddin Hussein announced in the morning that maritime patrols in the area would start on June 19 and that a date for air patrols would soon be set.

Indonesian Defence Minister Ryamizard Ryacudu also told The Straits Times before the luncheon that his country has invited Singapore to take part in joint Sulu Sea patrols.

Speaking through a translator, Mr Ryamizard said Indonesia has explored some "potential involvement of Singapore to join this effort", and that there is a "high possibility" of Singapore doing so.

According to Dr Ng, the ministers also discussed other security issues such as the US' engagement in the Asia-Pacific and North Korea's nuclear drive.

There were few questions from defence ministers on American commitment to the region, which Dr Ng attributed to what he felt was US Defence Secretary James Mattis' convincing reassurance during his speech in the morning.

"There is a consensus that the US message is very clear, not only from the point of view of the messaging but the messenger. The fact that you have Secretary Mattis there, a well decorated veteran of operations who understands how militaries work and the use of military force to engender peace and stability," he said.

There was also agreement that a nuclear-free Korean peninsula should be pursued, even as the threat has risen because of the increase in North Korean missile testing and nuclear capability.

"The instability on the Korean peninsula weighed heavily on all ministers' minds," he said, adding they realised the issue is now a global one because of the potential harm direct missile strikes and the aftermath can cause.