Poland captain Robert Lewandowski urged his team to improve, despite taking a step towards the 2018 FIFA World Cup in Russia with a 3-1 win over Romania.

Lewandowski scored a hat-trick as Poland moved six points clear atop Group E in Warsaw on Sunday (AEST).

The Bayern Munich forward opened the scoring in the 29th minute before completing his treble in the second half to move level alongside Cristiano Ronaldo as the leading scorer in European World Cup qualifying with 11 goals.

Lewandowski, however, was not completely satisfied with the victory after Romania finished strongly and pulled a goal back via Bogdan Stancu.

"Personally I'm happy, but for me what matters most to the team. We won and we have taken another step towards the Russian World Cup," Lewandowski said.

Poland has won five of its six matches to top the standings ahead of Montenegro and Denmark.

Romania is a further four points adrift after Sunday's (AEST) result.

"It will be difficult to maintain concentration. It's like driving a car, the most difficult are the last kilometres. The most important for us are now are the two games in September, and to win them will take us closer to promotion.

"It only depends on us, what will be. Therefore, we must be vigilant. The sooner we qualify, the better. Especially if you look at the last 20 minutes against Romania, which were not sensational."

Lewandowski continued: "Romania played very defensively. After the third goal we lost it. I do not know why. Maybe fatigue after a long season.

"In any case, such things cannot happen with the 10th team in the FIFA rankings."