The ministers and representatives of the 12 participating countries in the ambitious but controversial Trans-Pacific Partnership (TPP) talks failed to reach an agreement on Tuesday after four days of negotiations in Singapore.

But the ministers said in a joint statement that they made " substantial progress towards completing the Trans-Pacific Partnership".

"Over the course of this meeting, we identified potential ' landing zones' for the majority of key outstanding issues in the text. We will continue to work with flexibility to finalize these text issues as well as market access issues," they said.

The ministers said they intended to meet again in January next year.

Participants in the talks said that "there was a sense that the United States wanted to get the job done" in the last round of the talks before the end of the year and that the negotiators "have been working from early morning until late in the night."

US Trade Representative Michael Froman said that the ministers touched on "literally dozens of significant outstanding issues."

However, there are still additional works to be done before the agreement can be concluded, one of the participants told a press conference at the end of the four-day ministerial meeting.

The latest round of TPP talks had been considered important as leaders and senior trade officials of the United States have been pressing for the conclusion of the TPP talks by the end of the year.

The meeting carried on with its tradition of negotiating behind closed doors. The participants include trade ministers and representatives of 12 countries such as Singapore, the United States, Australia, Brunei Darussalam, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru and Vietnam.

The TPP has been envisioned as "a high-standard, comprehensive and forward-looking trade agreement that aims to address the challenges of the modern economy," according to a statement by the Ministry of Trade and Industry of Singapore at the start of the meeting. Some of the ministers at the meeting described it as "a 21st-century agreement."

However, the ambitious free trade pact has been controversial, too, as it covers not only traditional trade and investment issues, but also areas such as intellectual property, labor rights, environment and state-owned enterprises.

Some have said it is the economic pillar of the United States' pivot to Asia, which, with its pronounced increased military presence, has led to increased suspicion over its intention.

Market access is understood to be one of the key outstanding issues, and some of the countries have urged for the talks to be pragmatic.

There is only a minimal amount of information available on the agreement being negotiated, but WikiLeaks published what it said was part of the text of the agreement under discussion.

The United States is understood to have proposed demanding text for certain chapters on patent protection that will potentially cater more to the interests of multinational pharmaceutical giants. Advocacy groups and non-governmental organizations have voiced concerns, saying that this could hinder access to generic drugs for the poor in the developing countries.

The TPP was originally an initiative led by Singapore, New Zealand, Chile and Brunei Darussalam in 2005, but it has been dominated by the United States after it joined the talks in 2008. Japan joined the TPP talks earlier this year.

The current talks began in March 2010.

The trade pact is believed to potentially have an influence on the regional and even the world trade beyond the participating countries with its demanding text based on the origin of goods.