First-half goals from Shinji Okazaki and Riyad Mahrez secured a hard-fought 2-1 win over Club Brugge at the King Power Stadium on Tuesday.
The result, coupled with Porto's 0-0 draw away in Copenhagen, means the English champions are certain to finish top of Group G.
Now Ranieri wants his squad to concentrate on domestic duties, with Leicester sitting just two points above the relegation zone.
They have conceded 20 goals in 12 Premier League games and managed just one league win since the start of October.
"It's unbelievable. Top of the group. I'm very happy and proud for my players. I'm very happy for the chairman, the fans and everybody. Another journey – it's unbelievable," Ranieri said.
"It's important to top the group but our mind must now go to the Premier League. I must push my players for the Premier League.
"Whoever we play will be a fantastic team, but now we go back and we must play at the same level as we have done in the Champions League as we have shown what we can do."
Leicester cruised into a two-goal lead against Brugge, yet were forced to hold on in the closing stages after Jose Izquierdo scored a fine solo goal for the visitors early in the second half.
"It's very strange. We were playing so well. We had 70 per cent possession and had a lot of chances," Ranieri said.
The experienced Italian coach has now steered five different clubs through to the last 16 of Europe's premier club competition.
Finishing top of the group will see Leicester avoid some of the heavyweights at the start of the knockout stages, although Ranieri does not really mind.
"I just hope to be there in good condition. That's important. The name is not important. It's another fairytale," he added.