WASHINGTON - American opponents of TransCanada's Keystone XL pipeline are stepping up their line of attack.

New emails obtained by the environmental group Friends of the Earth suggest a cosy relationship between a TransCanada lobbyist and employees at the U.S. State Department, which will decide the fate of the controversial pipeline in the coming weeks.

The emails prove the approval process is hopelessly corrupt, Friends of the Earth says, since they show evidence of complicity between TransCanada and U.S. officials.

The emails are the second batch to be released by the group under freedom-of-information legislation.

They reveal a senior State Department official fishing for invitations to a Fourth of July party for TransCanada officials this year at the U.S. Embassy in Ottawa, and sharing details of Secretary of State Hillary Clinton's meetings with the company.

The official cheers on TransCanada in its quest for approval with a "Go Paul!" Paul Elliott is TransCanada's top lobbyist in Washington, D.C., and was a key official in Clinton's failed bid to win the Democratic presidential nomination in 2008.

A State Department official is denying the emails reveal a pro-pipeline bias.