Crystal Palace have rejected a bid of £18m from Everton for the forward Yannick Bolasie, with the south London club holding out for a fee closer to £30m.

Ronald Koeman has been searching for additions to his squad since succeeding Roberto Martínez as manager in June but so far has signed only the midfielder Idrissa Gueye for £7.1m from Aston Villa and the Holland goalkeeper Maarten Stekelenburg this summer.

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With the Dutchman keen to strengthen his options in attacking areas, Koeman has identified Bolasie as his primary target after being impressed with his performances for Palace in the Premier League since their promotion in 2013. The Democratic Republic of Congo forward has made 79 appearances and scored five goals in the league as Palace have enjoyed their longest unbroken spell in the top flight since the early 1970s.

However, after the manager, Alan Pardew, signed Andros Townsend for a club record £13m, Palace could now sanction Bolasie’s sale in order to free up funds for a new bid for the Liverpool striker Christian Benteke. A £25m offer was rejected last month, with Jürgen Klopp adamant the Belgian will not be allowed to leave for less than the £32.5m they paid Aston Villa last season.

It is understood that Everton intend to make a renewed offer of around £20m, although it is unlikely that will be enough to persuade Palace to allow a player signed for just £350,000 from Bristol City in 2012. A report last week suggested that the 27-year-old was considering his future at Selhurst Park following the arrival of Townsend, who will also compete with Wilfried Zaha, Bakary Sako and the young winger Sullay Kaikai for a starting spot.

“I don’t comment on reports or stories,” said Palace’s assistant manager Keith Millen on Monday night after a 3-0 friendly victory over Farnborough.

“The fact is Yannick is with us and he is a big part of what we’re about. He has worked hard in training and speculation with players isn’t great this time of year. It’s unsettling but we have to try and deal with it. Until a deal is done he’s in the squad and ready for the West Brom game.

“We do not want to lose any players. We want to strengthen in the attacking areas, we need more goals in the squad and we are working hard to try and get more goals brought into the club. Yannick is a key part of that, whether he plays wide or as a 10 and we want to keep all our best players.”

West Ham are also believed to have registered an interest in signing Bolasie, although that now appears unlikely after they completed the £20.5m purchase of Swansea’s Andre Ayew.

Everton are also considering a renewed bid for the Swansea captain, Ashley Williams. A £10m offer for the 31-year-old was rejected last week but he remains a target for Koeman.

Meanwhile Palace have also registered an interest in signing the Ghana international Emmanuel Agyemang-Badu from Udinese. The 25-year-old defensive midfielder almost joined Stoke last summer and is also wanted by West Ham, Middlesbrough and Sunderland.