Outsource giant Serco has played down weekend reports it wants to buy US homeland security firm SRA International.In a statement Sunday evening, the UK firm said it was "not in any discussions regarding any major transaction at this time," though it did not say whether it had been in discussions prior to the announcement.Stories at the weekend suggested Serco had made an offer worth $2bn for SRA and had been mulling a £500m rights issue to help pay for the deal. The US firm rebuffed this approach, while there also were rumours of possible interest from BAE Systems, which like Serco, is keen to boost its US homeland security presence.Shares in SRA have risen 27% so far this year on speculation it could be a bid target.