Istanbul - Turkish President Recep Tayyip Erdogan said on Monday that Ankara would close its border with Iraq's Kurdistan region over an independence referendum and threatened the Iraqi Kurds with blocking their key oil exports.Despite enjoying strong ties with the Kurdistan Regional Government (KRG), Turkey fears Monday's vote could stoke separatist aspirations among its own Kurdish minority. Erdogan also hinted the Turkish military were ready to act if needed."Entrance-exit will be closed" at the Habur border crossing, Turkey's sole land border crossing with Iraq, Erdogan said in a speech in Istanbul.He said there were currently crossings allowed to the Iraqi side only but travel would be closed in both directions this week. He added: "After this let's see through which channels they will send their oil through... and who they will sell to. The valve is with us. It's finished the moment we close it."Iraqi Kurdish oil is exported through Turkey and its southern Ceyhan port, a key economic lifeline for the region.Erdogan said "irrespective of the result, we see this (referendum) as null and void and say it is illegitimate".Erdogan said Turkey was ready to take "all the steps" needed "on political, economic, trade and security fronts" and appeared not to rule out military action.Last week the Turkish armed forces began a military drill in the region around the border town of Silopi including 100 military vehicles, which continued with reinforcements this weekend."The armed forces did not take necessary steps in Silopi for no reason. There is no compromise," Erdogan promised.All options on tableErdogan warned the KRG of a cross-border operation by the Turkish army similar to that taken in northern Syria last year against the Islamic State group and Kurdish militants.Dubbed Euphrates Shield, Ankara supported Syrian opposition fighters in northern Syria clear territory from IS and stop Syrian Kurdish militia from joining up their northern cantons. The operation -- begun in August -- was completed in March.Turkey views the YPG and the PYD as "terror groups" linked to the outlawed Kurdistan Workers' Party (PKK), which has waged a separatist insurgency inside Turkey since 1984 and has its rear bases in northern Iraq."The northern Iraqi administration must take a step backwards. We will absolutely not allow a federal state. In the same way, we will not allow a terror state formed in Syria," said Erdogan. Recalling the lighting offensive that kicked off Euphrates Shield, he added: "As I said, we can come unexpectedly in the night.""We did this with Euphrates Shield. All the options are on the table right now," Erdogan said, adding: "In Iraq, when necessary, we will not shy away from taking these types of steps."'Protect the Turkmen' Voting also took place in the city of Kirkuk - shared with Arabs and Turkmen - where control is disputed between the Kurds and the federal government in Baghdad.Turkish Foreign Minister Mevlut Cavusoglu later warned any intervention against "our Turkmen brothers" would lead to an immediate military operation by Ankara."It is our most natural duty to protect the rights of the Turkmen," quoted by state-run news agency Anadolu.The Turkish parliament on Saturday had approved overwhelmingly a one-year extension of a mandate to use troops abroad in Syria and Iraq.Premier Binali Yildirim said Ankara would take steps with regard to air space "before it is too late" although flights to northern Iraq from Turkey are reported to be running as normal.But he added: "Our citizens should not worry, we are not going into a war. They (Turkish measures) are pinpoint operations."