Deporting EU nationals who are not working and halving the length of time others can claim benefits are among proposals, she said.
She spoke out about "heated" rows with LibDem colleagues as the mainstream parties try to beef up their stance to counter the growing threat from Ukip.
She conceded that David Cameron's aim of reducing net migration to the tens of thousands "has become more difficult" after it was revealed the number was going up.
"We still have that aim of the tens of thousands. But of course it has become more difficult and net migration is too high," she said.
"That's why I want to continue working to bring it down. In those areas we can control - that is, immigration from outside the European Union - everything we have done as a Government has been having an impact."
She confirmed that deporting people who exercised the right to come to the UK to work but could not find a job was among measures being considered.
Work and Pensions Secretary Iain Duncan Smith said he wanted to introduce new laws this year to restrict benefit claims to three months.
The length of time EU migrants must wait before claiming was extended from three months to six, with the Tories and Labour indicating that could be made longer.
"You'll have to wait three months and you'll only be able to claim for three months. Then it's bye-bye," Mr Duncan Smith said.
Asked about Mr Duncan Smith's plans, Mrs May said: "We will look at that timing.We haven't yet got agreement across the coalition to do that. But these are the sorts of measures we keep looking at because in immigration you can never stand still."
The Office for National Statistics said net migration increased to 212,000 in the year to December, from 177,000 the previous year.
This was driven by an increase in EU citizens immigrating to the UK from 158,000 to 201,000 year-on-year.