An worker on offshore oil rig 'Marjan 2' in March 2003, located in the Persian Gulf, Saudi Arabia.

Saudi Aramco is working to secure billions of dollars in cheap loans from banks seeking to strengthen their ties with the oil giant before its stock listing, banking and export credit agency (ECA) sources said.

Citigroup (Citi), Standard Chartered and Sumitomo Mitsui Banking Corporation were advising on the transactions, which two sources said could raise at least $5 billion to $6 billion, all with ECA-backing.

The bid to raise funds is the latest indication of Saudi Arabia's push to ensure what could be the world's biggest initial public offering (IPO) goes ahead in 2018, despite market speculation that sale plans might be delayed or even shelved.

The loans will offer slim returns - probably less than 1 percent a year - but the sources said the banks hoped to position themselves for more work as the kingdom proceeds with selling up to 5 percent of Aramco in an IPO that could value the firm at $2 trillion.

For its part, Aramco wants to leverage its balance sheet before the IPO, after which it could face higher costs because, once listed, it would cease to be a solely state-owned entity benefitting from cheap funds available to sovereign borrowers.

ECAs offer loan guarantees and sometimes financing to help remove political and other risks facing exporters, encouraging trade and lowering the costs of international business.