The INMO Executive Council will meet today to discuss further industrial action on top of the five days already announced.
Industrial action is planned for Tuesday and Thursday next week (February 5 and7), and for three days the following week (February 12, 13 and 14).
37,000 nurses staged their first 24-hour strike on Wednesday leading to thousands of appointments being cancelled.
The union says the government has not made any "serious proposals" to avert more strikes and say they have been forced into this position.
The INMO says it will plan for further action if it does not get government support.
Director of Regulation and Social Policy Edward Matthews says he hopes it does not come to that.
"At the moment, we're concentrating on making the voice of nurses and midwives heard, we want to make sure the dispute is safe but is effective, and we'll consider options after that as is necessary," he said.
"But none of that should be necessary, the government should intervene to ensure the clear case being made by nurses and midwives is addressed in a fair and reasonable way within the confines of the national agreement which exists."
Meanwhile, 6,000 members of the Psychiatric Nurses Association (PNA) have implemented an overtime ban.
Following yesterday's industrial action, PNA members are set to hold further action next week on February 5, 6 and 7, as well planning to escalate this to a three-day strike on the same days as the INMO.
