Foreign Secretary William Hague, on the eve of a visit to long-isolated Myanmar amid signs of thaw in its relations with the West, urged the country's leaders to release all political prisoners and speed up political reforms.

Hague, making the first visit to Myanmar by a British foreign minister in more than 50 years, follows in the footsteps of U.S. Secretary of State Hillary Clinton, who visited Myanmar last month.

Hague said his two-day visit, starting on Thursday, was made possible by recent steps taken by Myanmar, including the release of some political prisoners, the easing of media curbs, opening a dialogue with pro-democracy leader Aung San Suu Kyi and allowing her party to take part in by-elections set for April.

I am visiting the country to encourage the Burmese (Myanmar) government to continue on its path of reform, and to gauge what more Britain can do to support this process, he said in a statement.

Hague, the first British foreign minister to visit the former British colony since 1955, starts his visit in the capital NayPyiTaw where he will hold talks with President Thein Sein and ministers.

He then travels to Yangon to meet Suu Kyi, the Nobel peace laureate whose British husband Michael Aris died in 1999. Hague will also meet ethnic leaders and former political prisoners.

Hague said Myanmar needed to take further steps that would have a lasting impact on human rights and political freedoms.

We hope to see the release of all remaining political prisoners, free and fair by-elections, humanitarian access to people in conflict areas, and credible steps towards national reconciliation, he said.

Myanmar has launched radical reforms since a civilian government was allowed to take power last March after almost half a century of rule by the military.

However, British officials do not see the reforms there as irreversible yet. At the moment, we are moderately optimistic but in recent history there have been a number of false dawns, one said.

Hague will hold out the prospect that Britain would push for the easing of European Union sanctions on Myanmar if the country makes substantial progress against three benchmarks, a British official, speaking on condition of anonymity, said.

The benchmarks are the release of all political prisoners -- estimated to number between 300 and 1,700 -- the holding of free and fair by-elections in April, and efforts to bring ethnic groups into the mainstream political process, he said.

The EU, in a move to encourage reform, slightly eased sanctions last April by ending travel bans and asset freezes on 24 civilian government officials. EU officials said in November the bloc was looking at whether Myanmar's reforms could justify a further easing of sanctions.

Officials said Hague would make an announcement on aid but gave no details. Britain has committed to spend 185 million pounds ($290 million) on aid to Myanmar between 2011 and 2015.

Britain channels its aid through international organisations or charities and not through the Myanmar government.

(Reporting by Adrian Croft; Editing by Roger Atwood)