Baghdad, Iraq (CNN) - Iraqi forces have tightened security in and around the southern holy city of Karbala as tens of thousands of Shiite pilgrims pour in ahead of this weekend's culmination of a revered mourning period.

Scores of pilgrims - who make the journey of foot - have been killed and wounded in recent days in attacks that have been blamed on Sunni extremists, who have routinely targeted the pilgrims since 2003.

The attacks coincide with a political deadlock that has the government divided along sectarian lines that is raising fears, in the wake of the U.S. military's withdrawal from Iraq, of a return to the sectarian bloodshed that nearly tore the country apart during the height of the war.

Prime Minister Nuri al-Maliki ordered security tightened following the attacks on pilgrims. Security forces have sealed roads and increased the number of checkpoints on the main roads leading into the holy city, a three-day walk south for pilgrims making their way from Baghdad.