A global migration agency is intensifying its effort to bring thousands of South Sudanese home from the north.

The International Organization for Migration, or IOM, says a convoy of 12 river barges carrying more than 3,000 southerners left the town of Kosti on Monday, bound for South Sudan's capital, Juba.

It says two IOM-supported trains arrived in South Sudan from Khartoum on Friday, carrying another 2,700 southerners.

The IOM says the trains are the first in a planned series that aims to bring 12,000 South Sudanese home from the northern capital by the end of the year.

South Sudan declared independence from Sudan in July. Thousands of South Sudanese living in the north are hoping to return home but have lacked the means to make the journey, which can stretch over 1,000 kilometers.

Sudan has given South Sudanese who live in the north a deadline of March 9 to legalize their residency or leave.

The IOM says many southerners in and around Khartoum have been living out in the open.

The migration effort comes at a time of rising tension between Sudan and South Sudan. Each side has accused the other of supporting rebels on the other's territory, and South Sudan's President Salva Kiir has accused Sudan of planning an invasion.