MANILA (AFP) - Philippine President Benigno Aquino held a surprise meeting in Japan with the country's top Muslim rebel to address growing concerns over delays in implementing a peace deal, their aides said on Thursday.

The meeting took place on Tuesday on the sidelines of a peace conference in Hiroshima where Mr Aquino and Moro Islamic Liberation Front (MILF) chairman Murad Ebrahim were guests, presidential spokesman Edwin Lacierda said.

A rebel spokesman said the MILF sought the meeting to raise concerns over delays in implementing an accord signed in March, after a draft law that is crucial for a final peace was not passed by Congress this month as planned.

Mr Lacierda divulged few details of the meeting, other than it was a 15-minute encounter and that they talked about the planned law that would create an autonomous Muslim region in the southern Philippines.

"It is safe to assume that the draft Bangsamoro Basic Law was discussed," Mr Lacierda said.

Muslim rebels have been battling for independence or autonomy in the southern islands of the mainly Catholic Philippines since the 1970s, with the conflict claiming tens of thousands of lives.

The MILF, with 10,000 armed followers, is the biggest rebel group and its signing of the accord has raised hopes of an enduring peace in the south, despite other breakaway groups still vowing to fight.

The pact made the MILF and the government partners in a plan to create the autonomous region by mid-2016, when Mr Aquino is required by the constitution to stand down.

A commission composed of rebel and government nominees drafted a "basic law" for the autonomous region, and it was submitted to Mr Aquino in April as part of a timeframe to have Congress pass it by June.

Mr Aquino, however, is still reviewing the planned law. His aides have said this is because he wants to ensure it withstands the scrutiny of the legislative body.

MILF deputy chief Ghazali Jaafar told AFP on Thursday that Mr Murad had sought out the meeting to "express our concerns over the delays", and had been partly appeased by Mr Aquino's response.

"We accepted explanations given, but any further delays could impact on the full ratification of the deal," Mr Jaafar said, while emphasising that MILF fighters remain "deeply concerned" about the slowdown.

He said Mr Aquino must marshall his allies in Congress to pass the law this year.

Mr Aquino, in his Tuesday speech before meeting Mr Murad, highlighted a similar encounter between the two in Japan in 2011 that built momentum in the peace process.

"Trust was established between brothers, and a genuine dialogue was possible," Mr Aquino said.