The US Friday called for "free, fair and credible" elections Saturday in three provinces in Sri Lanka including the north, once a Tamil Tigers bastion.
The US embassy said it had seen reports of an attack on the residence of a candidate, supporters and members of a domestic elections monitoring NGO Thursday evening.
The embassy said it was concerned over the attack, and said it should be "transparently and independently investigated" and the perpetrators brought to justice swiftly.
"We call on all parties to refrain from violence and observe a peaceful, open and transparent electoral process, one that is free, fair and credible," it said.