OSLO (Reuters) - Norway’s $1 trillion wealth fund’s process to blacklist firms for producing too much greenhouse gases has taken longer than expected, but recent clarifications from its ethics watchdog could mean a decision is near, a top official told Reuters.

The fund is the world’s largest sovereign wealth fund, with stakes in over 9,000 companies worldwide.

It is forbidden from investing in firms that produce nuclear weapons or landmines, or are involved in the production of tobacco, among other criteria.

Carbon emissions became a criteria for exclusion from the fund in 2016 and last year the Council on Ethics recommended “a small handful” of firms be excluded for producing too much greenhouse gas emissions in either the oil, cement and steel sectors.

Since then recommendations have been under review with the board of the central bank, which oversees the fund.

“We sent a letter to the council for further interpretation of that criteria and we just now got an answer, so the process is moving forward,” Deputy Central Bank Governor Egil Matsen told Reuters.

“We will deal with the reply internally in the bank during the next weeks and months, but I can’t say exactly how long it will take,” Matsen said, adding that the dialogue with the council had been extensive.

He said the main difficulty had been to define and limit this new type of criteria based on the companies’ conduct or behavior, rather than their products.