Shares of energy companies operating in Iraqi Kurdistan plunged on Thursday and two U.S. companies evacuated some staff as Islamic State militants advanced within 45 kilometers (28 miles) of Arbil, a key hub for oil producers. Companies that produce oil in the semi-autonomous region, including Oslo-listed DNO ASA, as well as London-listed Gulf Keystone Petroleum and Genel Energy saw double-digit declines in their share prices as the radical Sunni militants seized at least three more towns near Kurdish-controlled territory. While the companies said their oil fields continued to operate, there are growing concerns about the security risks faced by firms active in Iraqi Kurdistan, seen as a beacon of relative stability in the region.

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Shares in DNO fell as much as 24 percent and Gulf Keystone Petroleum was down 13 percent at their lowest point, before paring losses.

"Investors are now taking into account the risk,'' ABG Sundal Collier analyst Oeyvind Hagen said, adding it remained difficult to judge how far into the Kurdish region the Islamists could penetrate. Gulf Keystone said it had increased security at its flagship Shaikan field, Iraqi Kurdistan's largest, but added that production and trucking operations were continuing safely. Read MoreRussia, Iran strike oil deal--with no details DNO operates the Tawke field, in which Genel is also a partner, which produces around 120,000 barrels of oil per day. Other Kurdistan oil producers including London-listed Afren and Toronto-listed Oryx Petroleum Corporation, which operates the fields closest to the militants advance, declined to comment. Shares of Oryx were down as much as 11 percent in Thursday trading. Chevron and Exxon Mobil said on Thursday they were evacuating staff from Kurdistan as the militants advanced. The companies have been drilling and testing new wells in the region. Exxon said little about its Kurdistan holdings at an analysts meeting earlier this year. As recently as last week, Chevron executives told investors they were ``very encouraged'' by initial Kurdistan well results.