ISLAMABAD—Saudi Arabia, a key U.S. ally in the Middle East, had high-level contacts with America’s most deadly adversary in Afghanistan, the Haqqani network, according to purported Saudi diplomatic cables released by WikiLeaks.

The documents, which couldn’t be independently verified, say the Saudi ambassador to Pakistan met in 2012 with Nasiruddin Haqqani, the chief fundraiser for the jihadist group who has been on a United Nations terrorism watch list since 2010.

In the meeting, Mr. Haqqani requested medical treatment in Saudi Arabia for his father, Jalaluddin Haqqani, the founder of the terrorist organization, the diplomatic correspondence says. The documents indicate the elder Haqqani carries a Saudi passport.

The Saudi embassy in Islamabad wasn’t available to comment, and Afghan officials didn’t immediately respond to the allegations. Senior U.S. officials said they were reviewing the reports but they had no immediate comment. Earlier this month, Osama Nugali, a Saudi foreign ministry spokesman, said that many of the documents published by WikiLeaks “have been clearly fabricated.” But he also said that the documents “didn’t give any information other than that formerly announced by the ministry” on international or regional issues.

Washington’s ties with Saudi Arabia have been strained for years over U.S. concerns that funding for the Afghan insurgency and other terrorist groups in the Middle East comes from donors in Saudi Arabia and other Gulf nations. Successive U.S. administrations have been careful not to accuse Gulf governments of complicity in sponsoring terrorism, however.