A group of Gulf Keystone Petroleum shareholders have increased their stake in the Kurdistan-focused oil explorer above the 3% threshold at which they have to notify the stock market, the group said on Tuesday.The group, which includes the Joyce M. Kuok Foundation, Kerry Asset Management, Kuok Khoon Hua, Natalon Company, Shield Group, and the Zheng Ge Ru Foundation, all of whom are represented by Morgan Stanley Securities, bought close to 900,000 shares.The shares, worth £622,000 at Monday's closing price, took the six shareholders' holdings to 3.02%, Gulf Keystone said.The oil firm has endured a tough 2014, falling almost 60% since the turn of the year, despite managing to secure a main market listing and starting commercial production.In March, Gulf Keystone's share in the Shaikan field was limited to 163m barrels, after a report confirmed reserves of just 299m barrels on the site.The firm insisted it had only drilled 25% of the 109 wells it intends to use for Shaikan and that future wells were likely to increase reserves, but that did not prevent the company from racking up half of the year's price slide in just 24 hours.Last week the group's share price reached its lowest level in four years after security was increased at its operations in the Kurdistan province of Iraq, amid conflicts between Kurdish forces and Islamic State (IS) militants.Gulf Keystone shares were up 4.85% to 73.66p at 14:20 on Tuesday.DC