Manchester United scouts were reportedly at the Estadio Dragao last week during Porto's 3-1 win over Bayern Munich in the Champions League to monitor striker Jackson Martinez.

According to the Daily Mail, the Colombian forward is keen to move away from Portugal in the summer and his current employers are willing to listen to offers for the front man. But have made it clear that only offers around the £30m ($44.6m) mark will be considered.

Louis van Gaal is desperate to strengthen his strike force during the summer transfer window after seeing his forwards struggle to score goals on a consistent basis this campaign. Wayne Rooney is United's top scorer despite playing most of the season in the midfield, while main striker Robin van Persie and on-loan forward Radamel Falcao have struggled for form and fitness all season.

Reports suggest that the latter two will be offloaded at the end of the current campaign, thus making way for new arrivals in the summer.

United will not have a free run at the striker in the summer as Premier League rivals Arsenal are also interested in signing the striker with Arsene Wenger an admirer of the Colombian forward. But the Old Trafford club could be ahead in the race after sending scouts to watch him last week.

However, apart from Martinez, the Dutch manager has also been tracking Burnley's Danny Ings, who is available for £5m and Paris Saint-Germain striker Edinson Cavani, but the French club have made it clear that he is not for sale.

The report also claims that Van Gaal is interested in Aston Villa forward Christian Benteke, but the Red Devils are yet to make an approach for the forward, and Tim Sherwood has made it clear that he is willing to sell the striker if he indicates that he wishes to play elsewhere.

Meanwhile, Martinez, who played a starring role for Porto in their shock win against Bayern Munich in the first-leg of the quarter finals understands that the tie is nowhere close to being over and that his team will have to dig deep to dispose of the 2013 Champions League winners at the Allianz Arena.

"We know it will be tough and we will have to suffer, however, as the first leg, but we have good players capable of hitting on the break," Martinez said, as quoted by Tutto Mercato Web.

"The team is not dependent on my play. Things work together when you are fighting for a goal. I think that Bayern will keep an eye on all of our players, and not just me. We know that the race will be completely different from that to 'Estadio do Dragao'," the Colombian international added.