"Everyone knows how much I like Dortmund, but I also want us to have a powerful team which has a powerful approach," Hummels told German magazine Kicker.
"I have had a few conversations recently, which were very relevant for me to get a general overview, but I haven't made a decision about my future."
The 26-year-old Germany international has a Dortmund contract until 2017.
But the World Cup-winner has long been courted by United, whose coach Louis van Gaal is seeking a defensive leader with the Premier League giants reported to be willing to meet Dortmund's 35 million euro (US$37.9m) price tag.
"I often think that I definitely want to move abroad, but then there are other days where I say I don't need it," added Hummels.
"Basically, I think a foreign move would be good both for both personal and footballing development, so it could well transpire that I'd someday like to move abroad.
"It's definitely not the case that I am saying I will definitely go.
"When there is a decision from me in which direction things are heading, I'll talk about it openly.
"I'm not a fan of those who claim they are staying, but behind the scenes have already secretly settled the matter."
Dortmund host defending German champions Bayern Munich on Saturday and have moved up to mid-table in the Bundesliga after a disastrous set of results left them bottom of the league at the start of February.
They have a 12-point gap to bridge if they are to qualify for the Champions League next season and are 31 points behind current leaders Bayern.
Source : AFP