O'Neill said the Rangers striker, 31, contacted him on Sunday night to inform him of his surprise absence.

The manager added Lafferty had "ruled himself out of the two games".

He said: "Our focus is the players who are with us. I'll address the situation after the international period."

Northern Ireland face Austria in Vienna on Friday and Bosnia in Sarajevo on Monday, needing to pick up points after the opening home defeat by the Bosnians last month.

Lafferty started Northern Ireland's Nations League opener to earn his 68th cap and is one of the most experienced members of O'Neill's squad.

But O'Neill said Lafferty - who was benched for Rangers' match with his former club Hearts at the weekend - had taken the decision himself not to travel.

The Rangers forward has scored 20 international goals although he hasn't netted for his country since the 4-0 World Cup qualifier win over Azerbaijan at Windsor Park in November 2016.

Northern Ireland have not lost any of the 17 international games in which Lafferty has scored in.

His absence could present an early opportunity for Rotherham striker Kyle Vassell, who has been called into O'Neill's squad for the first time at the age of 25.

Lafferty has scored four Rangers goals since returning to the club from Hearts in August.