Scotland were sloppy after the break and were punished just before the hour as Yuri Zhirkov got ahead of Stephen O'Donnell as a square ball was turned home.

Scotland goalkeeper David Marshall kept the score down as the home side struggled to regain their early momentum.

Clarke said: "We felt we prepared well, as our start showed we probably had. We got on top and got ourselves in front and then, for a reason I have yet to determine, we stepped off the game and allowed Russia to dominate us and impose their style of play on us."

Both goals and further chances came from Scotland giving away the ball.

"It's something we have to address," Clarke said. "We spoke about it after the game. That's the biggest disappointment.

"We gave the ball away quite cheaply. We seemed to get very nervous after we took the lead for some reason, whether we thought we had to sit back and defend and see the game out.

"We had too many possession losses; too nervous on the ball and not enough threat going forward after the first 15-20 minutes.

"Russia started the second half better than we did, that's for sure, and put us on the back foot again. I thought we had weathered the storm and then lost possession and lost the second goal. A lot of players were out of position, getting caught on the halfway line. That was a really poor goal to lose.

"Towards the end we huffed and puffed a bit but didn't really look like scoring."

Clarke added: "If you look at the start of the game, we have good players. Why we allowed that game to drift away from us, whether it's a mental thing or a quality thing, we will need to assess."

The defeat leaves Scotland six points adrift of their second-placed opponents halfway through the campaign, with the visit of leaders Belgium on Monday and a trip to Moscow next month to follow.

Clarke said: "I spoke about the three difficult games we had coming up and that's three points away. So Monday night is beginning to look like a game that we need to take something from, whether it's one point or three, to give ourselves a realistic chance in the group."

Russia manager Stanislav Cherchesov felt his side responded well to McGinn's opener.

"I'm very satisfied with the way we played," he said. "After conceding the first goal, we actually played as we planned.

"We took some time to get into the game and for games at this level you have to do that from the first second, but in the end we did the job.

"I'm very happy we won and showed our character."