U.S. customs officials said they would not strictly enforce new identification requirements at land and sea borders with Canada that go into effect Monday, because of business leaders' concerns about the impact on trade and travel.

Customs and Border Protection officers will issue warnings to most people who lack the correct documents and use discretion in detaining people for questioning, said Jayson Ahern, acting customs commissioner.

The United States "will have a flexible enforcement policy on June 1," he said. Eventually, "we'll get to a point where" full compliance will be required, he said.

The new requirements are part of an initiative begun by the Bush administration and continued under President Obama to make it harder for terrorists to enter the country.

U.S. security officials have said the U.S.-Canada border, which is crossed by 400,000 people daily, is porous, potentially offering a prime entry point for terrorists.

Ahern estimated that as many as 90 percent of those crossing will have the right documents on June 1.

Customs officials took a similar approach last year when they began requiring travelers to provide proof of citizenship. While business and industry officials praised that effort for minimizing delays, they said it was unwise to impose new rules during an economic crisis.

"Any delay, any bottleneck, any legitimate traveler not able to go across to service a piece of equipment or for a legitimate business purpose is a problem," said Shaun Donnelly, senior director for international business policy at the National Association of Manufacturers, a Washington-based lobbying group.

Some of those fears may be unfounded because companies that frequently do business across the border will be prepared, said Christopher Sand, a senior fellow at the Hudson Institute, a policy research organization in Washington.

"Truck drivers will be fine, businesspeople will be fine," he said. "Where you'll run into trouble are families on vacation" and others who don't make the trip often.

Most of the impact of the new requirements will be felt along the northern U.S. border. Mexican citizens already are required to show a passport and visa or a border-crossing card to gain entry.

Currently, the United States asks for a driver's license and a birth certificate at the U.S.-Canada border to prove identity and citizenship.

As of June 1, customs officials will ask U.S. and Canadian citizens to show instead a passport, a passport card, a special driver's license that is hard to forge, or cards issued to frequent travelers.

U.S. citizens also can present a military or merchant marine document if traveling on official business. Children under 16 will have to show only a copy of their birth certificates.

Those traveling by air into the United States have had the requirements since 2007.