ASHGABAT, Turkmenistan (AP) — The energy-rich Central Asian nation of Turkmenistan has suspended shipments of natural gas to Iran, which it says has not paid for deliveries since 2013.

Turkmenistan's Foreign Ministry said in a statement Tuesday that Iran's failure to make payments in a timely fashion hindered Turkmenistan's ability to operate its gas transportation infrastructure. Gas deliveries were halted on Jan. 1.

Turkmenistan has been exporting gas to Iran under an agreement signed two decades ago, but occasionally raises its prices during the winter. In 2006, it suspended shipments and demanded a nine-fold price increase. Iran eventually accepted the higher prices for a short period.

In 2016, Russia stopped buying gas from Turkmenistan, which is now left with China as its sole customer.