UPDATE 5-EU looks at toughening Myanmar sanctions

* EU ministers to look at strengthening sanctions

* Some say only Asian states can influence military

* Activists say more could be done (Recasts with quotes from Czech presidency)

By David Brunnstrom

BRUSSELS, May 18 (Reuters) - EU ministers agreed on Monday to look at how sanctions on Myanmar might be toughened over its treatment of opposition leader Aung San Suu Kyi, but some said only Asian nations could influence the military government.

Suu Kyi went on trial on Monday, facing up to five years in jail after Myanmar's government said she had broken the terms of her house arrest in connection with an incident involving U.S. citizen John William Yettaw, who is alleged to have spent two days in her house this month.

"We have ... asked the EU institutions to examine ideas or proposals to strengthen or re-inforce measures," Czech Foreign Minister Jan Kohout said after a meeting with EU counterparts in reference to existing sanctions, which include visa bans and asset freezes on the military government and its backers.

The ministers also agreed on the need to step up dialogue with the Association of South East Asian Nations to encourage them to use their influence with Myanmar,Kohout, whose country holds the EU presidency, told a news conference.

British Foreign Secretary David Miliband said Suu Kyi was facing a "show trial", adding: "We ...need to make sure that the Burmese regime understands fully the risks that it's taking."