Iraqi intelligence officials offered the AP the letters, as well as the first known photograph of the Nusra Front leader, Abu Mohammed al-Golani, the head of one of the most powerful bands of radicals fighting the Syrian government in the country's civil war.
The officials said they obtained the information about al-Golani after they captured members of another al-Qaida group in September. They spoke on condition of anonymity because they weren't authorized to speak to journalists.
"I was told by a soldier that he observed some of the workers of the U.N. and he will kidnap them. I ask God for his success," read an excerpt of a letter given by officials from Iraq's Falcon Intelligence Cell, an anti-terrorism unit that works under Prime Minister Nouri al-Maliki.
The officials said other letters planned the kidnapping and killing of other foreigners, and Syrian and Iraqi civilians.
One U.N. worker was kidnapped for eight months in Syria and was released in October. Another two dozen U.N. peacekeepers were briefly held this year. It's not clear if those abductions had any relation to al-Golani's letters.
Syria's uprising began with peaceful protests, but it turned into an armed uprising after Assad's forces cracked down on demonstrators.
Since then, hard-line Islamic brigades have emerged as the strongest rebel forces in Syria, chiefly among them the Nusra Front.
Under al-Golani's leadership, it has dominated rebel-held parts of southern Syria, and it is a powerful fighting force in the Damascus countryside and northern Syria, with an estimated force of 6,000 to 7,000 fighters.
Al-Maliki's Shiite-majority government is considered a quiet ally of Syrian President Bashar Assad. The officials may have released the letter excerpts to underscore the dominance of al-Qaida in Syria.
The intelligence officials did not where they found the al-Qaida fighters who handed over the documents. They also would not say when the letters were written, though they said it represented a tiny sample of a large cache of documents.
The officials couldn't explain why the letter excerpts were in a sloppily written, grammatically incorrect version of an Arabic dialect used across the Levant. It is believed that al-Golani was an Arabic teacher before he rose through al-Qaida ranks, and typically hard-line Muslims try to write in classical Arabic.
It may have been that an aide was writing down al-Golani's speech. Arabs typically speak in dialects that are often quite different from the classical Arabic.
"The claim by Iraqi intelligence that Jolani and by extension, Jabhat al-Nusra, have been behind an explicit policy of kidnapping U.N. workers should be treated with some suspicion," said Charles Lister, a prominent analyst of Syria's militant groups. He referred to the Nusra Front by its Arabic name. "While it might well be true, elements within Iraq's security services have a clear interest in portraying jihadists in Syria and Iraq in a highly negative light."
Little is known about al-Golani, including his real name. He is believed to be 39 years old. The photograph suggests a man in his thirties.
Al-Golani is a nom de guerre, indicating he was born in the Israeli-occupied Golan Heights.
A Syrian native, he joined the insurgency after moving to Iraq.
He advanced through al-Qaida's ranks and eventually became a close associate of Abu Musab al-Zarqawi, the Jordanian-born leader of the militant group al-Qaida in Iraq.
He eventually returned to Syria shortly after the uprising against Assad began in March 2011, where he formed the Nusra Front, first announced in January 2012.
The group gained prominence in April after Golani rejected an attempted takeover of the Nusra Front by another rival al-Qaida group, now known as the Islamic State of Iraq and the Levant.
Iraqi intelligence officials said it was members of ISIL who gave them the information about al-Golani.
Associated Press writers Bassem Mroue and Diaa Hadid in Beirut contributed to this report.