Carter, on a rare trip by a Western dignitary, was greeted at an official ceremony at Pyongyang airport by North Korean vice foreign minister and nuclear envoy Kim Kye-Gwan, the official Korean Central News Agency (KCNA) said.
The Nobel peace laureate may leave Pyongyang on Thursday with Aijalon Mahli Gomes, an African-American who was jailed in April for illegally crossing into the North from China, the South's Munhwa Ilbo newspaper reported.
Kim Yong-Nam, the North's number two leader and nominal head of state, had a "cordial" talk with Carter before arranging a reception for his party at the Paekhwawon State Guesthouse, KCNA said.
Washington confirmed Carter was on a "private humanitarian mission to North Korea", which comes at a time of high tensions on the peninsula following the sinking of a South Korean warship in March with the loss of 46 lives.
"It's a mission to secure the release of Mr Gomes. But we don't want to jeopardise the prospects for Mr Gomes to be returned home by discussing any of the details," US State Department spokesman Mark Toner said.
Carter, now 86, made a landmark visit to Pyongyang in 1994 when the United States came close to war with North Korea over its nuclear programme. He helped defuse the crisis through talks with then-leader Kim Il-Sung.
"This visit will help defuse tensions and create an atmosphere conducive to resuming dialogue between the North and the United States," Yang Moo-Jin, a professor at the University of North Korean Studies in Seoul, told AFP.
Carter's trip comes as China is pushing to resume stalled six-party talks on dismantling the North's nuclear programme. Chinese nuclear envoy Wu Dawei will visit Seoul on Thursday after making a trip to Pyongyang last week.
Carter's mission is similar to the one by former president Bill Clinton last year when he secured the release of Americans Laura Ling and Euna Lee, television journalists who were jailed after wandering across the North Korean border with China.
The United States has repeatedly voiced concern about the health of Gomes, whom two American doctors and a US consular official visited earlier this month in a Pyongyang hospital.
Gomes, a 30-year-old former English teacher in South Korea and reportedly a devout Christian, was arrested in January. He was sentenced in April and fined the equivalent of 700,000 dollars.
His motivation for entering North Korea remains unclear. In Seoul, Gomes had joined rallies denouncing the North's dismal human rights record.
KCNA said in July that Gomes had tried to commit suicide and was being treated in a hospital.
He was "driven by his strong guilty conscience, disappointment and despair at the US government that has not taken any measure for his freedom," it said.
Relations between Washington and Pyongyang, already fraught over failed international efforts to halt the North's nuclear weapons drive, deteriorated after accusations that a North Korean torpedo sank the South Korean corvette.
Carter, who served as US president from 1977 to 1981, has gone on to make a career in diplomacy.
As an elder statesman he has been vocal about issues such as Israeli-Palestinian peace negotiations, humanitarian strife in Zimbabwe and the situation in war-torn Darfur.
Source: AFP American Edition