Citizens from 35 countries could be barred from boarding US-bound flights starting in March if they don't register online before flying, according to the Homeland Security Department.

The government hopes that registration will flag visitors with criminal records or immigration problems before they enter the USA. But travel groups worry that thousands of people could be kept from the country because they don't know about the requirement.

"We lose if these people don't come," said Geoff Freeman, senior vice president of the US Travel Association, a trade group. The association is urging the government to launch an aggressive publicity campaign.

The registration requirement affects 35 countries — including the United Kingdom, Japan and Germany— whose citizens can enter the USA. without a visa. In 2009, about 15 million people from those 35 countries visited the USA.

Online registration enables Customs officials to keep convicted criminals or illegal immigrants from boarding planes to the United States, said Customs spokeswoman Joanne Ferreira. "This makes sure travelers who do not require a visa do not pose any threat by traveling to the United States," she said.

The registration requirement is not connected to the Dec. 25 attempted bombing of Flight 253 over Detroit. Suspect Umar Farouk Abdulmutallab, a Nigerian, would not have registered because he had a visa, which requires a background check.

Congress mandated the registration program in 2007, and Customs launched it one year ago. The online forms asks biographical information and background questions, and can be completed a few hours prior to boarding.

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