Shares of London-listed oil firms active in northern Iraq fell for a second day as other field closures and staff evacuations became more likely in a region seen until now as relatively secure compared to the rest of the country.

Afren became the first company to announce that it was cutting oil production and withdrawing all essential staff from the field, with the company saying it “has taken the precautionary step to temporarily suspend operations at the Barda Rash field.” The company's stocks fell 4.5 percent, though it narrowed that decline later in the session.

The closure “underlines the severity of the security situation in Kurdistan and the potential risks for those operating in the region,” said analysts at Maribaud Securities.

U.K.-based Gulf Keystone Petroleum was down 6.2 percent, as the company said it had increased security at its flagship Shaikan field but confirmed production was continuing safely.

Among other oil producers cutting production as IS advanced, Genel Energy, operator of the two large Taq Taq and Tawke oilfields in Kurdistan, said it evacuated “non-core” personnel from fields in the region that are not producing oil. Taq Taq and Tawke are still operating, it said, and have been producing an average of 230,000 barrels per day (bpd) this week.

Islamic State fighters have advanced to little over a half-hour drive from Erbil, a city of 1.5 million that is headquarters of the Kurdish regional government and the local branches of many international businesses.

President Barack Obama said Thursday he had authorized limited U.S. airstrikes to blunt the onslaught of armed groups, which has heightened international fears of a humanitarian catastrophe.

IS considers non-Muslims and adherents to Shia Islam as apostates, and in many towns it has captured it has made a stark offer: Convert, flee, or die. Tens of thousands of members of Iraq's minority Yazidi sect have been driven from their homes and are stranded on Sinjar mountain.

Al Jazeera and wire services