Bangladesh emerged as one of the surprise packets of the tournament when they qualified for the second round for the first time since making their Cup debut in England in 1999.
They pulled off a major upset in their opening game when they whipped India by five wickets in Trinidad last month, the victory which virtually knocked the former champions out of the tournament.
Habibul Bashar's Bangladeshis proved that the win was no flash in the pan as they shocked top-ranked South Africa by 67 runs in a Super Eights match in Guyana.
"If we think this was a very good World Cup for us and we have finished our work, then it will be stupid," said Nafees.
"We are improving at the moment and we need to keep improving. We need to improve our standards of cricket so that in coming years we can be one of the top teams in the world. And that's our target."
Nafees believed the hard work was the key to Bangladesh's successes in the ongoing tournament, saying they had been playing good, competitive cricket for more than a year.
"We won a lot of games last year. We tried out new combinations and they worked well for us in this tournament," said the left-handed opener, a member of the side that won eight successive one-dayers before the World Cup.
"It was a continuous process and we have been playing very good cricket for the past one year and it paid off for us in this tournament.
"When we left the country, we expected to play in the second round. All the boys have put in a lot of hard work. They put things together and now we are in the second round."
Bangladesh began their Super Eights campaign on a poor note, losing by big margins against defending champions Australia and New Zealand before surprising South Africa. They also made England struggle for victory in the next match.
"We had bad games against Australia and New Zealand, but beat South Africa in the next game. Although we could not score enough runs against England, we put up a very good fight," said Nafees.
He said his side needed to put in a better batting performance in the last two games against debutants Ireland on Sunday and the West Indies on Thursday.
"We know we have not put up big scores, but our bowlers have been backing us very well. We are playing well as a team, but it will be fine when we can post big totals. It will also be good for our bowlers," said Nafees.
Bangladesh's main problem in previous matches was skipper Bashar and his deputy both were out of form. Bashar has so far scored 61 runs in seven matches and Nafees 24 in five.
"We hope he (Bashar) come back into form. I am also not getting too many runs in the tournament, but our young players are batting well at the moment," said Nafees.
"Form is a temporary thing, but we are working hard and trying to focus as much as possible. If we can stay a bit longer, I think we can score big."