While the southern and central fronts were at a stalemate, Iran began to focus on carrying out offensives in northern Iraq with the help of the Peshmerga (Kurdish insurgents). The Iranians used a combination of semi-guerrilla and infiltration tactics in the Kurdish mountains with the Peshmerga. During Operation Karbala-9 in early April, Iran captured territory near Suleimaniya, provoking a severe poison gas counter-attack. During Operation Karbala-10, Iran attacked near the same area, capturing more territory. During Operation Nasr-4, the Iranians surrounded the city of Suleimaniya and, with the help of the Peshmerga, infiltrated over 140 km into Iraq and raided and threatened to capture the oil-rich city of Kirkuk and other northern oilfields. Nasr-4 was considered to be Iran's most successful individual operation of the war but Iranian forces were unable to consolidate their gains and continue their advance; while these offensives coupled with the Kurdish uprising sapped Iraqi strength, losses in the north would not mean a catastrophic failure for Iraq.