The 28-year-old Scotland striker, who signed from Wolverhampton in 2014, reached his century of goals in the 3-0 home Europa League play-off win over Suduva last week and is keen to extend his stay at Parkhead and double that figure.
When asked about a new deal with the Scottish champions, Griffiths, on duty with Alex McLeish’s squad for the double-header against Belgium and Albania, said: “It is still ongoing. These things are never easy.
“It is almost completed now and hopefully in the next couple of weeks it will be done and dusted. Hopefully it is a nice long contract.
“Why would you want to leave a club like Celtic? You are playing week-in, week-out, in front of 60,000 fans, winning trophies every season.
“There are not many players who can say they have played for Celtic.
“I am in an elite number of players who have scored 100 goals and I want to extend that, I want to try to get to 200 now. If the contract gets over the line that will be my next target.”
Griffiths believes he will come up against the best defender in Scottish football should Celtic team-mate Dedryck Boyata feature for the Belgians at Hampden.
Boyata has returned to Brendan Rodgers’ side following his absence after a bid by Fulham was rejected by the Parkhead club. The player said he was injured, a claim which was refuted by Rodgers.
Griffiths said: “There’s not many players up to the level of Dedryck’s standards in Scotland. Internationally that’s a different level. But if he does play on Friday night it will be a good battle. I try to stay clear of him in training. He is a great player and that’s why a number of clubs are wanting him. Since he came back we have had three clean sheets which is no coincidence.
“There was a lot of talk over what was going on behind closed doors but Dedryck showed his professionalism in coming in and playing to the standards that he can play and that’s why he got a bronze medal at the World Cup. We are delighted he is staying.”