The 33-year-old Klose, who is five goals short of Gerd Mueller's all-time German scoring record of 68 international goals, returned to action this month for his club Lazio after a five-week break due to a thigh injury sustained in March.
He is locked in a battle for a Germany starting spot with Bayern Munich's Mario Gomez, with the tournament starting in Poland and Ukraine on June 8.
"I had my concerns regarding Miro a few weeks ago but since I saw him back in training those worries have become smaller and smaller," Loew told a news conference in southern France where the Germans have set up their pre-tournament training camp.
"In just a short time Miro is again very dynamic and good. He has had a couple of days off with a back problem but he has trained again."
Klose will be playing against Switzerland in a friendly on Saturday with Gomez and seven other Bayern players joining the team later on Friday following their Champions League final defeat to Chelsea last week.
"Miro makes an extremely good impression but the next two weeks will be crucial for him as well. He will need to work. Now it is 60-70 minutes but we need him in top performance for 90 minutes. He must be able to complete the (training) programme," said Loew.
The coach, in his third tournament in charge, said Bayern players would need to catch up on tactical discussions after arriving more than 10 days after the rest of the squad.
"That the Bayern players come too late, we know it and what can you do?" said Loew. "Obviously it would have been great if they were here 10 days ago but that is the way it is now.
"They arrive today, they join training on Monday. Tactics are in the foreground and Bayern have to work a bit to catch up. We have made a lot of analyses and they have to catch up."
Germany take on Denmark, Portugal and Netherlands in the tournament group stage.