(UPDATE) The start of construction for a Japanese-financed subway in Manila could be pushed back to as far as December next year due to some constraints, an official said Tuesday.

• Duterte: Japan to help PH realize its infra dream

The government had said previously that construction of the P355-billion project would start by the first quarter of 2018, meaning it would be delayed by about 9 months.

But the Department of Transportation said the project was "on track" and groundbreaking for the project could happen by the fourth quarter of next year.

The DOTr said a portion of the subway could be completed by the second quarter of 2022, in advance of the full completion in 2025.

Jonathan Uy, assistant secretary for investment programming at the National Economic and Development Authority, said during a forum in Manila that many things need to be taken into account, such as the formalization of a loan agreement.

"You are familiar with the fiscal year of the Japanese, that most agreements and withholding loans are signed on the first quarter of any given year because this is when (the Japanese parliament) confirms the Cabinet's pledge," Uy said.

The official said it is still possible that the construction could start as originally scheduled.

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The new schedule is based on an estimated timetable for Phase 1 of the Metro Manila Subway Project given by the Japan International Cooperation Agency to the development authority, the state body that approves the country's big-ticket infrastructure projects.

Formal arrangements for the loan agreement were initially expected to be finalized when Japanese Prime Minister Shinzo Abe meets with Philippine President Rodrigo Duterte at the summit of the Association of Southeast Asian Nations in Manila next week.

Besides the loan agreement, Uy also cited procurement concerns and implementation constraints that could possibly put off the start of construction.

The initial phase of the mega subway, the first such project for the Philippines, will cover a stretch of about 23 kilometers from Quezon City to Paranaque. It is also expected to be used by 370,000 people daily.

• Metro Manila subway gets Duterte approval

The subway is projected to be completed in 2025, according to the development authority.

Separately, the Philippines and Japan are also discussing possible funding of a $197 million anti-flood infrastructure project in Cavite, a city south of Manila, as well as a railway project north of the metropolis.

The $4.27 billion railway will stretch from Malolos in Bulacan to Clark in Pampanga, according to the Finance Departmen