Illinois's attorney general has said county clerks statewide can began issuing marriage licences to same-sex couples, although the state law legalising gay marriage will not come into effect until June.
Democratic attorney general Lisa Madigan sent the letter in response to a request for legal guidance following a ruling in February from district judge Sharon Johnson Coleman.
Currently, the ruling on licences only applies to same-sex couples in Cook County.
In the letter, Madigan said other counties were not bound by last month's ruling, yet her office expects it to be "persuasive" to other courts addressing the issue.
Madigan stated her office would argue denying a marriage licence before the law is unconstitutional, if a couple were refused the document now.
Her statement comes after an observation from David Orr, Cook County clerk, on the day of the federal ruling. He previously stated same-sex couples who live outside Cook can obtain marriage licences as long as they marry in the area.
Mike Nerheim, a Lake County official, said they had no plans to start issuing same-sex marriage licences before the law takes place. According to Nerheim, questionably valid licences could cause problems with future divorce and probate cases.
He told the Chicago Tribune: " We just want to make sure that those licences are valid."
Officials from other counties in the Chicago area have not yet responded to requests for comment.
Same-sex marriage was made legal in the state in November 2013, making Illinois the 16th state to legalise the process in the US. Pro-LGBTQ legislation swept the nation, which saw Hawaii legalise gay marriage just two weeks earlier.
Pat Quinn, governor of Illinois, said: "Marriage equality is coming to Illinois. I look forward to signing this landmark legislation on November 20 and celebrating a big step forward with the people of Illinois."
President Obama also expressed support for the legislation in his home state. He said: "As I said in my Inaugural Address last January, our journey as a nation is not complete until our gay brothers and sisters are treated like anyone else under the law, for if we are truly created equal, then surely the love we commit to one another must be equal as well."