HANOI, July 18 (Reuters) - Vietnam's President To Lam has taken over the duties of Communist Party chief Nguyen Phu Trong who is focusing on an unspecified health treatment, the party said on Thursday.

The extraordinary move comes after months of speculation that Trong's health was weakening. The party did not clarify what treatment he is receiving and a spokesperson for the ministry of foreign affairs had no additional information to share when asked at a regular media briefing.

Lam, a former head of internal security, has been widely seen as a top candidate to replace Trong when his term ends in 2026. The party's powerful Politburo has tasked him with temporarily presiding over "the work of the Party Central Committee, the Politburo and the Secretariat," according to a statement from the party's central office.

"Looking at the current state, Lam has the highest chance to become the acting party chief until 2026," said Nguyen Khac Giang, expert on Vietnam at Singapore-based think tank ISEAS-Yusof Ishak Institute.

For Lam to be officially appointed as acting party chief, a new Politburo decision will be needed.

In its statement on Thursday, the Politburo called on the party, the army and the people "to have absolute confidence in the leadership of the party".

Though Vietnam officially has no paramount ruler, the party chief holds the most powerful position in the Communist-ruled nation. Trong, 80, has been in the post since 2011.

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