US President Donald Trump will meet Prime Minister Najib Razak in Washington next month, signalling a new phase in bilateral ties that have been clouded recently by the US Department of Justice probe into the affairs of state-owned investment firm, 1Malaysia Development Berhad (1MDB).

The White House said Mr Trump would welcome Datuk Seri Najib on Sept 12 to celebrate the 60th anniversary of bilateral relations.

A White House statement yesterday said: "President Trump looks forward to celebrating the 60th anniversary of United States-Malaysia bilateral relations, and discussing ways to strengthen and broaden our bilateral relationship and expand regional cooperation with one of America's closest partners in South-east Asia."

The official visit represents a major political boost for the embattled Malaysian leader, who is struggling to lift his political prestige, hit mostly by the 1MDB scandal, at home.

Mr Najib already enjoys close ties with China's top leadership, particularly with President Xi Jinping, who is set to visit Malaysia in November.

Diplomats said the warming relations with the Trump administration could present Kuala Lumpur with a unique role to play in ties between the two superpowers as they jostle for dominance in the region.

According to Malaysian government officials, Mr Najib will be in the US from Sept 10 to 12, during which he will hold talks with politicians on both sides of the divide - Republicans and Democrats - in Washington. He will also visit New York on Sept 11, where he will pay his respects at the National September 11 Memorial and Museum to mark the 16th anniversary of the World Trade Center attacks.

The announcement of Mr Najib's trip to Washington comes at a time when other little-publicised backroom engagements between the two countries are taking place.

Malaysia's director-general of the powerful National Security Council, Tan Sri Zulkifeli Mohd Zin, is currently in Washington for talks at the invitation of US National Security Adviser H.R. McMaster.

Diplomats said Mr Zulkifeli, the former armed forces chief, is part of the "advance party" for Mr Najib's visit and his talks with Lieutenant- General McMaster today would centre on Malaysia's growing role in the US strategy to neutralise North Korea. US intelligence has pinpointed Malaysia as a favoured location for North Korea and its proxies to hold secret meetings, do business to generate foreign currency, and use its ports and airports as transit points for defence-related shipments.

Secretary of State Rex Tillerson drafted Malaysia as a key ally in Washington's efforts to counter Pyongyang during a visit to Kuala Lumpur this month. He received assurances from the Najib administration that Kuala Lumpur would dismantle the financial infrastructure of private North Korean firms that operate in Malaysia, and also severely restrict any use of Malaysian ports and airports as transit points for military-related shipments.

Shortly after Mr Tillerson's visit, the US State Department removed Malaysia from the Trafficking in Persons watch list, which ranks governments on their perceived efforts to combat human trafficking.

The announcement of President Trump's invitation to Mr Najib next month is another diplomatic gesture of appreciation, diplomats said.

Bilateral ties became frayed during the tail-end of the Barack Obama administration when the US Justice Department launched a criminal probe into 1MDB over allegations of money-laundering. The department has also filed civil lawsuits in a bid to seize over US$1.7 billion (S$2.3 billion) in assets allegedly acquired with misappropriated 1MDB funds.

How the Trump administration deals with the probe will be crucial for Mr Najib, who must call an election before the middle of next year.