WASHINGTON (Reuters) - The U.S. State Department said on Monday it had advised its embassies to limit the help they give visiting presidential candidates Sens. Barack Obama and John McCain so as to avoid violating U.S. law and policy.

Secretary of State Condoleezza Rice cabled the advice to all U.S. diplomatic posts on Thursday, the day that Obama left on a trip that has taken him to Afghanistan and Iraq and will include stops elsewhere in the Middle East and in Europe.

Undersecretary of State Pat Kennedy told reporters it was coincidental the cable was sent as the Illinois Democrat left and said its advice echoed that given when McCain, an Arizona Republican, visited Canada, Colombia and Mexico this year.

The cable bars U.S. diplomats from setting up meetings for the candidates, arranging receptions or public events and from using official "funds and resources, beyond a de minimis level, to support a political trip."

However, it said the embassies could assist the candidates' Secret Service protective details and provide "de minimis" help on logistical matters, for example giving the trip organizers information on how to make local travel arrangements.

U.S. diplomats are also allowed to help Obama and McCain with anything related to their Senate work and to provide classified briefings to ensure they were up to speed on recent developments and U.S. policy positions.

The cable draws a distinction between trips by official congressional delegations which are paid for by the U.S. government, and those that are privately funded, such as campaign-related foreign travel.

Congressional delegations can receive far more help than lawmakers who travel privately or for campaign purposes.

U.S. officials said that McCain's visit to Iraq in March was as part of a congressional delegation, while subsequent visits to Canada, Colombia and Mexico were campaign-related.

Obama traveled to Afghanistan and Iraq this week as part of a congressional delegation but the rest of his trip, which is expected to include stops in Israel, Jordan, Britain, France and Germany, will largely be campaign-related.

Under U.S. law, there are restrictions on the partisan political activities of federal employees to prevent them from exerting undue influence on elections. There are also policies governing how federal workers may aid presidential candidates.

"It is imperative that in implementing these various requirements we treat both major presidential candidates evenhandedly," the cable said.

(Editing by Eric Beech and Cynthia Osterman)

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