WASHINGTON -- The Obama administration is pushing to protect Antarctica's fragile environment by imposing mandatory limits on the size of cruise ships sailing there and the number of passengers they bring ashore.

At a conference set to begin Monday in Baltimore, U.S. diplomats will propose amending the 50-year-old Antarctic Treaty. The move would seek to mandate, under international law, the current voluntary restrictions on tourism.

A U.S. document provided to The Associated Press by the State Department says the plan would "minimize the likelihood of marine oil spills" in the Antarctic and "ensure that tourism is conducted in a safe and environmentally responsible manner."

Secretary of State Hillary Rodham Clinton was to kick off the conference in Washington on Monday by hosting the first joint meeting of Antarctic Treaty signatories and the Arctic Council, which covers the northern polar region. More than 400 officials and observers are expected to attend from the Baltimore meeting, which runs to April 17.

The Baltimore meeting will mark the 50th anniversary of the pact's signing. Many consider it the first modern international arms control treaty because it says Antarctica cannot be used for military purposes and freezes sovereignty claims on its territory.

The treaty says Antarctica can be used only for peaceful purposes and guarantees freedom for scientific investigations. It sets out guidelines under which the continent can be protected. There are 28 member states and 19 observer countries and organizations to the accord.

The new U.S. proposal contains no specific enforcement mechanism or penalties for limiting tourist operations. But it would require signatories to the pact to ensure that Antarctic tour operators bar ships with more than 500 passengers from landing sites, restrict landings to one vessel at a time per site and limit passengers on shore to 100 at a time.

It would mandate a minimum of one guide for every 20 tourists while ashore, according to the documents.

Limiting tourist access to the continent has taken on urgency because of a surge in visits and recent cruise ship accidents, including two groundings in the just-finished 2008-09 season and the highly publicized sinking of a vessel in November 2007.

The International Association of Antarctica Tour Operators says visits have risen from 6,700 in the 1992-93 season to 29,500 in the 2006-07 season and 45,213 in 2008-09.

Members of the association first developed the restrictions and adhere to them voluntarily. Members are backing the U.S. proposal for the mandatory limits, which were first adopted by the Antarctic Treaty parties as recommendations in 2007.

"We follow them religiously," said the group's executive director Steve Wellmeier. He acknowledged that without mandated limits, enforcement is "an honor system to a large extent."